So hey, I've played this game ages ago and wanted to play again.
I', looking for some builds like tech(magic) assassin, battlemage, gunslinger, swordsman(tech or magic included). I'm more into tech in this game, but i like magic too. I would prefer human, elf, or half-elf. So any ideas?

I suggest you to play this game as a smooth talker,it is surprising to find out what you can do with a lot of charisma.

I've already tried doing this in my past gameplays

You could do something with a tech thief who plants dynamite, uses tranquilizer guns, and maybe experiment with some of the more exotic grenade schematics. Are there any backgrounds that actually give a bonus to fire damage?

I've tried to do this build once but got stuck on the Black Mountain Clan mines. Do you have any suggestions to get past that as a smooth talker?

My guess would be just to recruit a lot of followers and have them help you fight things. The Black Mountain Clan mines are probably the most annoying part of the game, by the way, so it's worth struggling through them to get to the rest of the game, no matter what character build you have. Even my pure melee fighter (who was otherwise really easy to play) had trouble there.