If you're lagged, that's going to suck no matter what. But as far as beating this guy, here are my tips. I fought the guy a bunch of times over and over with a friend. We kept encountering a bug where the boss would randomly heal to full life, so that's why we fought it a lot.
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All tips relate to stage 2. I find stage 1 very easy, but if you need help on that, let me know.

The tentacle smash attack from above... you should see a red circle appear below you. If you have nothing to aggro, it's going to happen several times. It is easier to just keep moving while he is doing this, imo. Fire the odd AoE attack to deal with mobs as you're moving. His arm attack that goes parallel to the floor and basically punches you (with the red outlines indicating where attack will be)... AVOID the middle area. That's bad news, generally. Stay on one side. I try and say closer to the end, but not the very end for very long. When he hits one area, generally the next attack won't hit the same area. So be prepared to walk over the last hit area on the next attack, and you probably won't be hit. Eventually he starts hitting two spots at once. If they are side by side, you'll likely need to do some rolling to get out of there.. try not to roll into middle.

The spotlight attack (white circle), of course, at all costs, run into it when you can. If you have the heal circle (whatever that spell is called), that';s a good time to drop that, right inside that spotlight area.

Try and kill the mobs ASAP, but it's also important to bring down the Lambach (boss) as quick as you can too. My buddy and I kept running into a bug where the boss would heal full part way through the fight, so we found putting him down as quick as we could was best. There is a moment where he pukes up a mob of monsters.. at the very start he's invulnerable for a few secs.. right after that up until before he spits them out, hit him with everything you can.

Don't forget your aspect transformation. If you get in a pickle, pop that on really quick to go invulnerable briefly, then turn it off to conserve the bar. We used ours to kick bosses butt a few times when he was in that animation of throwing up the mob.

Hope that helps.

What @CBMurdoc said is on point. Watch the telagraphs and also don't spam health potion when you get into trouble. If you are generating health globs don't use them all at once, if you can help it. I usually use my aspect if I get into trouble or as a finish, when I get the boss down.

For the arms, attacks are telegraphed, just avoid the zones where it's gonna land. (the space bar is your friend!)
For the circle, as soon as it appears, rush to it, it's the safe zone, rush there then spam damage on the boss! (the space bar is still your friend!)
And if you feel really too weak, don't forget that you can spam the few last zones for a while, gain a level up or 2 and some gear then come back to kicking its ass! Good luck!

https://www.youtube.com/watch?v=PfvN5co9gx0&t=116s