Each player add 5000 HP to Merasmus's health. Unless the player can contribute 5000 damage in 90 seconds, he is essentially useless, or worse than useless, because he handicaps others.
When you fight Merasmus, Medic and Spy is, IMHO, completely useless, so is Pyro due to bosses not burning and Merasmus gibbing him once he tries to use his short range weapons, relegating him to role of chubby scout. Scout has very minor, situational prophunt ability, Heavy and Engineer are weak due to huge nerf on Mini/Sentry damage on bosses (seriously, they deal barely more damage than Sniper's SMG...), plus the fact Merasmus kills Engies easily causing Sentries to kill opposing team; Demo fails to contribute due to short range, non hitscan weapons (unless everyone on server goes demo, plants huge sticky trap where Merasmus appears and booms him once he shows up).
What that leaves? Soldier with Direct Hit does so-so, but headshotting Sniper, no matter what he uses, deals 1000-1500 damage on headshot, "buying" his share in Merasmus' HP in 4-6 shots, then leaving only core HP to deal with. Easily done in 90 seconds.