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So to put it in order:
Swirlix comes out first against Garchomp.
Garchomp will always use Earthquake.
Swirlix takes a hit, leaving it with one HP.
Swirlix can either use Endeavour to bring Garchomp down to one HP or it can use Cotton Spore to slow Garchomp down a bit.
If it uses Endeavour, it'll be hurt by Rough Skin and will faint.