Oh, and at 240 damage per shot, he needs 5-6 seconds of shots (or revolver getting huge accuracy minus when you don't let it cooldown) to equal 1 second of Sniper charge time, and that happens only when Merasmus is vulnerable - when you consider Sniper can do it all the time, at will, and he can bodyspam Merasmus for massive damage when he is vulnerable, from any point of the map...
Nope, Spy is useless.
Kritz medic can contribute 1 kritz ubercharge (8 seconds contributing maybe 2000 damage to BB Heavy's Minigun) during Merasmus stay, then he has to contribute 3000 damage using his 150 bullets (1500 damage before even counting Merasmus damage resistance...) from point-blank range (read - dead Medic long before then).
Congratulations, Herr Doctor, you just wasted everybody's time by not even doing 5000 HP.
I won't even comment Sniper damage - 1080 hp on Huntsman without worrying about falloff, 1554 on Machina - seriously, how is the Spy approaching that?
The trick with Sniper is to aim above Merasmus head, at just below his skull hat - since he floats, arrow/bullet seems to clip his body hitbox if you aim for the head, dealing much smaller damage, but even then, damage everyone else deals is puny in comparison. With headshots, it ceases to be a contest.
Btw, I killed Merasmus 15 times. See my backpack at number of spells if you don't believe me - and I have to say, every time we had less than 50% of snipers, Merasmus always went home. Always. And that was on server with 20 players firing only on Merasmus, zero PvP, full cooperation.