I'm guessing you're playing Pathfinder due to the mention of "Invulnerable Rager."

The real answer here is that your GM needs to step it up - if the players are built such that each of you can kill or incapacitate an enemy per round, then obviously the GM has to ensure there are enough enemies in each combat so that it won't end before everyone gets at least one turn of doing their thing.

But not all GMs have the skill or experience to make those kind of adjustments, so let's look at what you can do. The unfortunate answer is "not much" - Barbarians are damage machines and little else, so your options are basically "hit as hard as you can" and "hit somewhat less hard than that." Really the only other thing you can do is a combat maneuver of some kind, like trip, disarm or sunder, for some pseudo-battlefield control - the downside is that each of these has enemies they aren't very good against, so focus on whichever one you can get the most mileage out of. But again, the solution to this problem lies with your GM, there's only so much you can do without straight up not playing your character properly.