So, Giant is now at 55n70e20z? If so...
Conditional on breath weapon recharge:
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* If Ulmo's breath weapon is ready again this round, try the tranquility gas again, and then swim to 5n20e.
* Else ready a swim to 5n20e for as soon as the giant starts moving.
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Hmm... aquatic creatures should have a "swim-by attack", though granted that's not a feat I would have voluntarily chosen for Ulmo for the arena.