Ment
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Without acceleration, it's unlikely to work; the chances are slim. But if:
1) Position the Hydras closer to the gates.
2) As soon as the Hydras' turn comes, cast Haste on them (not earlier, of course, otherwise they will slow down).
3) Let the Hydras skip their turn (the Chaos Hydra's default speed is 7, while the Skeleton Warriors' is 5; even if the latter skip their turn, the former will move later, meaning the Skeletons won't be able to attack in the same round).
4) Have the accelerated Hydras emerge from behind the gates and attack the Skeletons.
5) Next round. On the turn of the Serpent Swarms (ideally, the first ones, otherwise after the Ghost Dragons), teleport the Hydras back behind the wall.
That's how I envision it, but the Hydras may suffer losses from the enemy undead, the Skeletons may block them, and there are many other drawbacks. But you asked for a fantasy scenario, so here it is.
1) Position the Hydras closer to the gates.
2) As soon as the Hydras' turn comes, cast Haste on them (not earlier, of course, otherwise they will slow down).
3) Let the Hydras skip their turn (the Chaos Hydra's default speed is 7, while the Skeleton Warriors' is 5; even if the latter skip their turn, the former will move later, meaning the Skeletons won't be able to attack in the same round).
4) Have the accelerated Hydras emerge from behind the gates and attack the Skeletons.
5) Next round. On the turn of the Serpent Swarms (ideally, the first ones, otherwise after the Ghost Dragons), teleport the Hydras back behind the wall.
That's how I envision it, but the Hydras may suffer losses from the enemy undead, the Skeletons may block them, and there are many other drawbacks. But you asked for a fantasy scenario, so here it is.