So...I've seen some talk on these boards about how "The freeware bosses are not equally difficult", which is true. Giving examples, people seem to list Nine-Toes as the easiest boss (good choice) and Super Meat Man as the hardest boss (I don't agree here).
Let's assume worst-case scenario--you're just doing a gold-farming run in v0.15 and have a Goblin Tinker whom you're not buying anything with. Let's assume you can get to level 8 (that's the level you reach just by killing most of the enemies without doing any fancy exp slingshotting). This gives you a 52 damage attack, and 32 damage fireball. Super Meat Man deals 48 damage, so you can attack him once, fireball him once, dealing 84 damage, and then explore 6 squares, healing to full (6*8=48), while Super Meat Man only heals 60 damage. So...for every six tiles you explore, you're dealing 24 free damage to him. Or, if you're lucky enough to find the HALPMEH glyph, it heals 24 HP at level 8, so you can spend that 6 mana to heal twice and attack for another 52 damage (instead of fireball for 32), meaning (52+52)-60 = 44 free damage for every six tiles you explore.
Now, there's 400 tiles on the map; maybe only 60 are left by the time you can pull this off. That's still 240-440 free damage. But heaven help the Meat Man if you're level 9 (more damage and more regen) as that would make it 300-700 free damage. And heaven help the Meat Man if you're level 8 but have a lot of tiles unexplored (say 120) as that's 480-880 free damage. Remember, all of this is before you do things like use your health and mana potions, before you use your mid-battle level-ups. The poor boss only has 954 HP. And all of this is a lot more effective if you're a class with real attack power, or you buy a few items from shops that buff your attack or defence.
Compare this to, say, Frank the Zombie. Frank deals a full 75 damage, which means at level 8 it takes almost 10 tiles to regen that blow. You can still sneak in a little damage with your regen fighting, but at level 8 it's like...5 damage for every six tiles you explore (whether or not you have the healing glyph) so you'll maybe get 50 free damage if you spend a bunch of effort on regen. So...you pretty much have to deal with Frank's full 636 HP every time. And your potions won't be as valuable against Frank as they are against Super Meat Man. Now, maybe you could use the poison glyph to take advantage of regen..oh wait no: Frank's immune to poison.
Compare this to, say, Tower of Goo. Did you want to use regen fighting with Tower of Goo? Good luck with that. Alright, so let's say you have 13 MP, and set yourself up for a mid-battle level-up, fireball the boss twice, level up, fireball twice more and then use your mana potions and fireball some more; even if this mid-battle level-up takes you from level 9 to level 10, that's still only 272 fireball damage--not enough to kill. Ok, how about a double-mid-battle-level-up from 7 to 8 to 9--that's 300 damage, which is also not enough to kill (but brings it into range of a physical death). I mean, don't get me wrong, if you have Pactmaker or Mystera or similar big mana bonuses, you'll crush Tower of Goo. If you have poison you'll crush Tower of Goo. But if you don't, I've found Tower of Goo to often be un-winnable.
And there's Gharbad the- Whoa! Yes, if CYDSTEPP spawns on the map (roughly 33% chance) then you auto-win. If it doesn't, well...Gharbad takes more fireballs to kill than Tower of Goo and isn't particularly afraid of the poison glyph. Sometimes effectively fewer if you have a first strike effect, to be fair.
Don't get me wrong--Super Meat Man is still among the harder bosses, but I'd maybe rank it around fourth-hardest. (Actually, I'm not sure whether I'd put SMM above or below Lord Gobb now that I think about it...).