Пандора
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Look. When you summon F. (again), it gets added to the summoned ones.
Now, specifically about the battle.
You summoned 3 F., the opponent killed 2, leaving 1. Now it's your choice.
If you summon, there will be 4. If you skip your turn, the opponent kills 1 F., and 2 are resurrected. You summon three more phoenixes with your (skipped) turn. In total, you get 5 instead of 4, and it's your turn again (you can) summon 3 more phoenixes.
After resurrection and addition, all phoenixes have the ability as if they hadn't died. (I'm 95% sure.)
I have a vague doubt... soon you'll be giving us advice :) And that's good.
But wouldn't it be better not to wait until you're killed, but to summon? Then maybe you'll kill. Besides, those you summon can also be resurrected if necessary.