As sayafrost mention, keep Garuda on Obey, not Sic. You will need to manually use her skills, but two points here on why Obey is better.

1) Garuda has a knock back skill that can anger most people due to knocking back the mob that is currently being tanked (usually doesn't work on bosses though.)
2) Garuda's final skill will double the time on your currently applied dots to the mob. If she is in Sic mode (ie she casts her spells whenever they are available) then she will use it as soon as possible. Meaning she casts it near the start of the fight where you probably have 1 maybe 2 dots up and it pretty much wastes her skill. Being on Obey, dot up the mob, then use her skill to double the dot timers.