Leech is too weak

This issue of enemies having leech resistance is very bad, it completely kills the leech mechanic, my bloodmage can't leech anything, not even my acolyte. It would be great if there wasn't this leech resistance mechanic

Agreed that leech resistance on enemies scales poorly. I have a post talking about how to fix and properly balance leech while also removing the leech resistance from enemies:

https://www.pathofexile.com/forum/view-thread/3755078

To over-summarize, my post says that leech resistance should be removed, but leech should not be affected by players' damage modifiers (base damage and gem level would still count) so that it will scale way more consistent and reliably.