Thanks a lot for your answer.
I just got the rob of the sorceress and her resistances did a big jump ! I'm now at 47% without changing anything else. I think I figured it out : I thought when you unlock a new ability via skillpoints, like the elemental resistance of ward matery, you gain the bonus for every points spend in skill. So, for me, ward mastery should have given 70% elemental resistance (10% / level). In fact, it's 10% per level from the needed level to unloick it, so 10% at level 7, 20% at 8, etc...
Am I wrong ? it's pretty confusing.
My main is a blademaster I played as a tank / mele dps till recently when I realized that with a bow and proper skills I could make a ton of damages without any downside. But for the most part of the game I had my tank with low damage and I played Nathalie as pure damage dealer, so her equipment is purely damage oriented without any resistance. I din't have any issue with that till recently (and in fact I still don't have, apart from the very big bosses of the game, the only ones able to kill my blademaster). For the most part I find the game very easy (I play in hard).