Jesus saves, takes half damage

[Last Pawn]

I'd have to say that you were on the track, but I no longer have access to the 3e venoms. That being said:

If the venom in question does straight damage, then you buy the damage that the victim takes when they save without "Resistible", and the damage they take if they don't save with "Resistible".

If the venom is an affliction then it's a little more clunky as you're buying two afflictions (one with at an extremely high level to be "irresistible", and one more likely to be resisted).

Saying the victim takes half damage if they resist is basically having a resistible attack that is in fact irresistible and doubles effect if the target fails to resist, essentially denying the victim a saving roll against the lesser portion of the attack. By my recollection there is supposed to be nothing in 4th ed that denies a target their resistance roll.