So lets say i have -20 enemy light res and my merc has -30 enemy light res.

We both hit the same target, will it stack or will we override eachother with each hit ?

Its applied individually and not shared at all

So if i have -50 enemy light res and my merc has -10, his -10 will remove my -50 and apply his -10 each time he hits ?

No.. your skills will work under -50 enemy res

And his attacks will work under -10

The -enemy res attribute applies to each individual independently

Oooooh i see, ty