That was one of the main reasons the romans organised their troops in 3 echelons. When the cohesion of the first echelon started to suffer, they would retreat behind the second echelon and reform, leaving the fighting to the second echelon. If THAT echelon was in danger of losing cohesion, they would retreat behind the 3rd, last echelon. The most experienced soldiers would be placed here, to ensure that this line WOULDN'T break cohesion in battle. When the other echelons had reformed, they'd reassume their frontal positions. This way, the romans could always have (relatively) fresh soldiers at the front, maintaining unit cohesion over extended periods of time.
Yeah, I already knew about the triari, velites and the hastati (and don't forget the princeps (cavalry) in the flanks ). But I'm not talking about a battle SO massive. I'm saying a simple big combat: a little more than a skirmish, a little less than an epic battle. I'd be happy with that.