Kinda hard question but here you go.

If I have the card Dark Cure already face up on the field when my opponent synchro summons One-Hundred Eye Dragon while Vennominaga the Deity of Poisonous Snakes is in the graveyard. My opponent only has 500 life points left. Would he lose? I know Dark Cure Implies that it targets, and even if it does, does the effect of Dark Cure resolve first. Oh and Nurse Reficule the Fallen One is face up on the field.

This question I had for a Dueling Network Admin, and he didn't know so I am asking you guys.

Thanks.

I don't see why he wouldn't lose. Is there something that you think is preventing him from losing? Because the Deity of Poisonous snakes' effect only applies onto itself, if you think that deity applies to other monsters too.

Chain Link 1: Dark Cure gives 1500 life to opponent.

During the chain: Nurse Reficule's continous effect happens and inflicts 1500 damage on your opponent.

If you mean that he banished Deity, he can't on summon. Dark Cure would activate on summon. Also, Hundred-Eyes Dragon can only banish level 6 and lower monsters for their effects.