Round table

Here's a question, dear players =)
At the beginning of the battle, there were 3+3 Archangels. By the end of the battle, there were 1+2 Archangels left. Neither the group with 1 Archangel nor the group with 2 Archangels could revive the adjacent Archangel (the Archangel doesn't have enough power to revive another Archangel). Then, the same thing happened with Hydras - 1 Archangel couldn't revive a dead Hydra.
Question: what criteria are needed for an Archangel to be able to revive, and who can it revive?