Get a free switch against fini, bulk up(make sure terrain is up so you can't get burned by a gambling scald) then click your super effective hits. Even if fini stayed in and kissed it will need a crit to be able to let latios finish everything. Zarude is going to get 2 leftovers recovery no matter what.
attacks
0 SpA Tapu Fini Draining Kiss vs. 80 HP / 0 SpD Zarude: 110-132 (29.6 - 35.5%) -- guaranteed 4HKO after Leftovers recovery
Possible damage amounts: (110, 110, 114, 114, 116, 116, 120, 120, 120, 122, 122, 126, 126, 128, 128, 132)
attacks
252 SpA Latios-Mega Ice Beam vs. 80 HP / 0 SpD Zarude: 240-284 (64.6 - 76.5%) -- guaranteed 2HKO after Leftovers recovery
Possible damage amounts: (240, 244, 246, 248, 252, 254, 258, 260, 264, 266, 268, 272, 274, 278, 280, 284)
attacks
+1 176 Atk Zarude Power Whip vs. 248 HP / 68+ Def Tapu Fini: 398-470 (116 - 137%) -- guaranteed OHKO
atacks
+1 176 Atk Zarude Darkest Lariat vs. 0 HP / 0- Def Latios-Mega: 414-488 (137.5 - 162.1%) -- guaranteed OHKO
371(Max HP) - 132(kiss max roll) + 46(2 turns of lefties recovery) - 284(beam max roll) = 1 Remaining HP left