General criteria: first of all, you have to find the previous monster multipliers easy when playing with your toon. (Typhoon Mod is quite useful to test this: if you find it quite easy, then your toon has good chances to succeed when playing M4ssboss3s..at least when fighting against common monsters).
And then, when fighting bosses a few criteria (You donīt need all of them, but at least two (3 would be the advisable quantity).
1) High damage attack. What I mean here is to count on at attack that allows your toon to kill a boss with 2 or 3 hits (a "harrower" or a "smiter").
2) a great AOE atttack always available (any of them: Markmanship, Ice Shards, and Ternion Attack; and all the possibilities that are given by a -100 recharge gear).
3) Strong on defense: there are a lot of possibilities here: damage absorbtion; high resistances (the resistances under M4ssboss3s doesnīt guarantee that your toon will stay alive, but low resistances usually means instant death here); 100% ctap+high DA, ADCTH, etc...
4) A ranged attack. I include this on the defensive part just because the is the most useful tactic on this mod, so use and abuse the static attack key. Is something (What I mean here is to attack in complete immunity...that key is quite important even in vanilla game, but while in vanilla game or even in softer multipliers I donīt like to use that to take advantage of inflicting damage in a total inmunity situation, sometimes in M4ssboss3s Mod you donīt have other choice) that I donīt like to use under other moster multipliers, but is the only thing that you can do in a lot of situations when playing at M4ssboss3s mod.
I include below the number of each boss in this mod (apply to each number a small margin of error):
Act I: Nessus (24), Polyphemus (24), Arachne (24), Gorgon sisters (24x3), Hydra (9), Alastor (24), Talos (9), Minotaur Lord (24), Megalesios (18).
Act II: Scarab (24), Statues under the sphinx (4 of each; and good news here: the numer of statues and canopies are the same, so once you have killed all the statues you can enter the room to take the "hand of eternity"), Nehebkau (24), Manticore (9), Aktaios (18), Sand Spectre (24).
Act III: Quimera (24), Barmanu (24), Yeti (24), Xiao, the giant peng (24), Bandari (24), Liche Dragon (9), Yaoguai (24), Ormenos (12).
Olympus: Typhon (24).
Act IV: Grayas (12x3), Charon (12), Cerberus (12), Undead Typhon (12), and Hades (12).