The archangel is a very intriguing unit, but for 550 points, I don't think it really makes the cut.
As a 550-pt unit, the archangel competes with the wyrms for utility. The archangel is a basic attack unit whose presence does not fundamentally alter the course of how the game is played, though arguably it's one of those units that requires a non-lethal means of removal, like martyrs, heretics, possessed, berserkers sometimes, paladins sometimes, etc. Currently, all the wyrms have specific roles:
Dracolich: The dracolich is three things - a powerful attacker (but defensive because of low range without levitation), a force multiplier because of the risen, and a suppression unit due to the -1 command thing. As a unit that gives yet more units, the dracolich fairs well with any unit that benefits from your creatures dying. Since those new units tend to be among the enemies to begin with, dracoliches also benefit greatly from assassins to replace the new risen. The best dracolich armies try to take advantage of multiple of the dracolich's roles.
Dragon: The dragon has one specialty - rapid dominance. Dragons don't heal, they are not able to enhance your army by special abilities, much less make it grow. All dragons are are tough enough to take a bit of an initial punch and fast and strong enough to very quickly reach and destroy the enemy. They're the most offensive unit in the entirety of Zatikon. Dragons have two actions and they fly 3 spaces. This gives them an effective range of 6 with fireball, or they can completely erase something within 4 of their position. The best dragon armies move too fast to allow the opponent to set up a combo, which is good because the dragon army doesn't stand up to attrition.
Hydra: The hydra is tough enough to get by, but its main advantages are that it's the cream of the crop for unit summoners and that it will DESTROY anything left within 3, and can hardly be blocked in this. Serpents make your army bigger and make hydras difficult to assault except from extreme range. The best hydra armies either enhance the somewhat lesser toughness of the unit or take advantage of the waves of chaff units it can create (or both--mourners).
Feathered Serpent: Feathered Serpents are 550 points of army enhancement. -1 to enemy damage makes even those armor 2 units tougher and healing everything at the end of your turn can combine with that to make opponents very frustrated. Finally, on top of all THAT resilience, you turn the first loss every turn as a delay instead. Army sustaining powers par excellance make this unit a major, MAJOR target for any enemy, and yet it can do all of its stuff from the back row and can fly to get away from enemies.
Now, getting back to the Archangel, it is fundamentally an attacking unit. To be effective, it must reach the enemy and then attack in melee to be of use. Two actions used for attacking at power 5 is pretty much how it works, since auto-killing murderers isn't quite so useful when you sunk more than half of your army into this one unit... any losses at all really hurt you. The archangel is range 4, so it cannot rush an opponent fast enough to prevent them from deploying and just starting their combinations, like the dragon can. Its big special ability is that it doesn't die. Like ghosts, the archangel returns to the castle whenever it's killed. Of course, with a deploy cost of 5, that's a serious delay for your army, even if it's not a permanent loss. Add onto that the fact that it has inferior range to the other primarily-attacking unit at 550 points and killing an archangel results in the archangel army losing several turns with it. Oh, and as a new unit each time, the archangel wasn't around to witness its own murder, so murderers on the board are cleared and the killing unit isn't susceptible as a murderer when the angel comes back for revenge.
Let's talk comparisons. The archangel is range 4 and can lay out two power 5 hits. Very good, just not worth 550 points. For comparison, a flying werewolf is also range 4 and lays out two power 6 hits, and it has the same resilience (minus the not-dying ability). You can have 6 of these in an army... I suppose my big point is that for an army's power to be so concentrated into one unit, that unit needs to be able to pressure the enemy.
The archangel's inherent defensiveness with significant time and effort before it returns means that it fails to pressure the enemy as much as a 550-point unit needs to. Add on the fact that like many units, it can be completely lost to a non-lethal take-down, such as from a witch, geomancer, priest, love potion, summoner, possessed, druid, rogue, enchanter, or magus, and as a flying unit it's vulnerable to shamen and guardian units, and you have anything but assurance that the 550 points is being useful.
How about some possible solutions? I have two possible solutions to think of. First, if the unit really is intended to be the sort of slow and steady defensive advance unit that it seems to be portraying, then make sure that its able to come back... or at least increase those chances. Make the archangel immune to spells (some sort of divine ward or something). I don't like that option quite so much because just negating other units draws away from the tactical choices of the game, making army construction more valuable than smart decision-making.
The second option is to make the archangel slightly more aggressive and to give it some semblance of army-preservation. Give the archangel the ability to move to an allied unit's current position and bounce that unit back to the castle. This enhances the archangel's ability to get into the fight and gives it the notion of being a guardian angel for your troops. Using this ability would preclude the ability to fly for the turn, as this is movement. Alternatively, for more of a command cost, you could allow the archangel to deploy as an assassin, doing the same thing, but requiring that the enemy kill it before it can protect another unit (and if they don't kill it, then there's an archangel running wild among their troops). The deploy cost of 5 would help to keep this one reasonable, though I'm not sure it's enough of a boost to make up for the angel's deficiencies.
As a 550-pt unit, the archangel competes with the wyrms for utility. The archangel is a basic attack unit whose presence does not fundamentally alter the course of how the game is played, though arguably it's one of those units that requires a non-lethal means of removal, like martyrs, heretics, possessed, berserkers sometimes, paladins sometimes, etc. Currently, all the wyrms have specific roles:
Dracolich: The dracolich is three things - a powerful attacker (but defensive because of low range without levitation), a force multiplier because of the risen, and a suppression unit due to the -1 command thing. As a unit that gives yet more units, the dracolich fairs well with any unit that benefits from your creatures dying. Since those new units tend to be among the enemies to begin with, dracoliches also benefit greatly from assassins to replace the new risen. The best dracolich armies try to take advantage of multiple of the dracolich's roles.
Dragon: The dragon has one specialty - rapid dominance. Dragons don't heal, they are not able to enhance your army by special abilities, much less make it grow. All dragons are are tough enough to take a bit of an initial punch and fast and strong enough to very quickly reach and destroy the enemy. They're the most offensive unit in the entirety of Zatikon. Dragons have two actions and they fly 3 spaces. This gives them an effective range of 6 with fireball, or they can completely erase something within 4 of their position. The best dragon armies move too fast to allow the opponent to set up a combo, which is good because the dragon army doesn't stand up to attrition.
Hydra: The hydra is tough enough to get by, but its main advantages are that it's the cream of the crop for unit summoners and that it will DESTROY anything left within 3, and can hardly be blocked in this. Serpents make your army bigger and make hydras difficult to assault except from extreme range. The best hydra armies either enhance the somewhat lesser toughness of the unit or take advantage of the waves of chaff units it can create (or both--mourners).
Feathered Serpent: Feathered Serpents are 550 points of army enhancement. -1 to enemy damage makes even those armor 2 units tougher and healing everything at the end of your turn can combine with that to make opponents very frustrated. Finally, on top of all THAT resilience, you turn the first loss every turn as a delay instead. Army sustaining powers par excellance make this unit a major, MAJOR target for any enemy, and yet it can do all of its stuff from the back row and can fly to get away from enemies.
Now, getting back to the Archangel, it is fundamentally an attacking unit. To be effective, it must reach the enemy and then attack in melee to be of use. Two actions used for attacking at power 5 is pretty much how it works, since auto-killing murderers isn't quite so useful when you sunk more than half of your army into this one unit... any losses at all really hurt you. The archangel is range 4, so it cannot rush an opponent fast enough to prevent them from deploying and just starting their combinations, like the dragon can. Its big special ability is that it doesn't die. Like ghosts, the archangel returns to the castle whenever it's killed. Of course, with a deploy cost of 5, that's a serious delay for your army, even if it's not a permanent loss. Add onto that the fact that it has inferior range to the other primarily-attacking unit at 550 points and killing an archangel results in the archangel army losing several turns with it. Oh, and as a new unit each time, the archangel wasn't around to witness its own murder, so murderers on the board are cleared and the killing unit isn't susceptible as a murderer when the angel comes back for revenge.
Let's talk comparisons. The archangel is range 4 and can lay out two power 5 hits. Very good, just not worth 550 points. For comparison, a flying werewolf is also range 4 and lays out two power 6 hits, and it has the same resilience (minus the not-dying ability). You can have 6 of these in an army... I suppose my big point is that for an army's power to be so concentrated into one unit, that unit needs to be able to pressure the enemy.
The archangel's inherent defensiveness with significant time and effort before it returns means that it fails to pressure the enemy as much as a 550-point unit needs to. Add on the fact that like many units, it can be completely lost to a non-lethal take-down, such as from a witch, geomancer, priest, love potion, summoner, possessed, druid, rogue, enchanter, or magus, and as a flying unit it's vulnerable to shamen and guardian units, and you have anything but assurance that the 550 points is being useful.
How about some possible solutions? I have two possible solutions to think of. First, if the unit really is intended to be the sort of slow and steady defensive advance unit that it seems to be portraying, then make sure that its able to come back... or at least increase those chances. Make the archangel immune to spells (some sort of divine ward or something). I don't like that option quite so much because just negating other units draws away from the tactical choices of the game, making army construction more valuable than smart decision-making.
The second option is to make the archangel slightly more aggressive and to give it some semblance of army-preservation. Give the archangel the ability to move to an allied unit's current position and bounce that unit back to the castle. This enhances the archangel's ability to get into the fight and gives it the notion of being a guardian angel for your troops. Using this ability would preclude the ability to fly for the turn, as this is movement. Alternatively, for more of a command cost, you could allow the archangel to deploy as an assassin, doing the same thing, but requiring that the enemy kill it before it can protect another unit (and if they don't kill it, then there's an archangel running wild among their troops). The deploy cost of 5 would help to keep this one reasonable, though I'm not sure it's enough of a boost to make up for the angel's deficiencies.