If Dhelmise does "Giga Drain" and the Defending Pokemon only has 10 HP left, does Dhelmise still heal itself for 30 damage?
Yes, because the 30 damage is still done, even though it is more than enough to KO the Defending Pokemon.
Giga Drain





x2
Giga DrainHeal from this Pokémon the same amount of damage you did to your opponent's Active Pokémon.


Powerful SpinThis Pokémon can’t attack during your next turn.
If Dhelmise does "Giga Drain" and the Defending Pokemon only has 10 HP left, does Dhelmise still heal itself for 30 damage?
Yes, because the 30 damage is still done, even though it is more than enough to KO the Defending Pokemon.