I'm playing single player + plugy (mule purpose).
Now the barb is level 45, farming Travincal to gain 1 or 2 more level.
So far diamonds work well against trash, but I still need lots of potions to tank bosses.
I have wasted too many points in combat skill tree before level 36 (otherwise killing bosses with merc would have driven me mad), so currently I'm lacking of both physical and magical power. I fight minor bosses with concentrate/berserk and use wc against mobs(in fact, merc does a better job than me).
Need 2 skill rings to boost wc damage a bit I guess.
Things I learned as a low level player:
1. Double swing is good, but frenzy sucks if you only want to spend 1 point.
2. Money = better equipment (gem, socketing, crafting, etc.)
3. Gem shrines ftw
4. Get rid of mobs on the way, just in case you can't beat the dungeon boss.