I'm using the Belzebub mod. I'm near the end, I just have Map of the Stars and Diablo quests left. I'm lvl 27 Barbarian and when I got to Doomlord Ga'mar I just found it impossible because he keeps spawning allies. Should I just focus on leveling up more? I started using shield and 1 handed weapon instead of the 2 handed axe. It's my first time playing Diablo so I'm learning as I go. Any advice would be appreciated.

Well you probably should have started with vanilla Diablo then.
The Belzebub mod is pretty nice, but it has balance issues. So yeah, try leveling up and farming for items a bit in easier boss areas like Lazarus or Izual, maybe.

Ok. Any tips on how to best use Barbarian class?

Well I finally beat it. After getting past Ga'mar I was able to take everyone else out in 1 try. Beating Diablo was super rewarding. The really shitty thing is after beating Diablo I couldn't find Albrecht's body and get the soul stone for the ending. I looked all over and couldn't find it, not sure if it was a glitch or what. So I had to watch the ending on a fking Youtube video. Besides that garbage the game was great, hard but great.

I saw the playthrough of Belzebub on YouTube months ago.

Diablo's Soulstone should appear in your inventory after defeating him (it looks like the Bloodstone). As far as I know, right-clicking on it should trigger the ending.