When using Hoopa GX's "Devilish Hands GX" attack, can I keep choosing a Pokemon that would already be Knocked Out by previous choices, or do I have to choose a different Pokemon once I have done enough damage to KO the first one?
You make all 6 choices before doing any damage; so if you wish, you can do more damage than you need to a Pokemon in order to knock it out.
Source: Team Up FAQ (2019-01-18), TPCi Rules Team (2019-01-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM9_166
SM9_166
SM9_187
SM9_187
SM9_96
SM9_96