Well technically weaken reduces the enemy damage Reduction and enemy resist isn't the same as player resist. 25% resist for enemies means entirely 25% damage Reduction to that specific element and with higher resist, that 25% can be reduced to possibly even -% values or so.
Although greatly resist on enemies means reduce all your damage of that element by 75% so the enemy has 75% damage Reduction to that element .
Whenever you have the enemy reduce your damage, it multiplies if you have a Frozen build or poison build because the nominal 1 gets reduced and that basically has a cumulative reduction if your attacks multiply.
Immune enemies make your damage to something like 0 but with weaken 100% , the immune can be nullified . Basic almost 100% damage Reduction to that specific element.
Player resist works differently though since it really uses resist formula but it is dependant on level and enemy level but weaken can work on players indeed as tested. 40% weaken is the cap in PvP but players won't really have enough resist to justify weaken in arena and most damage Reduction comes from AR and sets. Although I do remember when players had so much resist (before they know much about other forms of damage Reduction) that weaken definitely helped but it's no longer the case.
I mean maybe enemies do actually use the resist formula but as they get higher in level, they get exponentially higher resists but I doubt that since they always have a set damage Reduction to their element.
For example, greatly resist always reduces your damage to only be 25% as powerful to enemies and 75% of it has been reduced . If you attack an enemy with a weakness element , you always gained 25% damage and the enemy lost their 25% damage Reduction .
Weaken makes it so the enemies cant even reduce your element damage given enough of it but it has a % chance of activation like with players. That's why weaken isn't best on players since it doesn't activate 100% but for enemies, you can get that high of a number.
My hypothesis: if enemy has 25% Reduction of damage to the element they are strongest in and you have 100% weaken.
25%×(-100% weaken)= -25% damage Reduction to the element or no Reduction and instead they suffer 25% more damage . If greatly resist happened, they'd end up adding even more damage for you instead of reducing with weaken . Or it could be -125% if it was additive but either way, having high enough weaken nullifies enemy defenses of resists and makes immunes useless. It's also possible that effective is less useful with higher weaken since they reduce less damage so the damage gains are negligible.
I have seen Midlumer weaken Vs ignore resist guide though and it is accurate , if negative enemy resists exist and they probably do.