When fighting medusas, circle them (this requires at least 35-40 speed) They fire where you were, not where you'll be. This makes them easy kills than trying to fight them up front. In cases where they get alot of helpers, do what pretty much any class has to at this juncture and... run away.
I suppose I can list tactics for some other monsters that allows a melee character to fight them, since I favor melee.
Ent Gods are the opposite of medusas and fire where you'll be, not where you are (unless you aren't moving/running right at/away from them) Trick them out to get close and get some shots in. Slime gods are the same, but are much tighter in their AI with it so it can be difficult.
If you have 40 speed or more, you can successfully circle a sprite god once you get close and not get hit
White Demons you cannot fight. This issue has been voiced on various channels and forums. There is nothing you can do against these but run away.
Ghost Gods you need a crystal sword to touch otherwise just leave them alone.
Beholders aren't bad if your monitor is bright enough to see while blinded. The blind shots do no damage. Their shots are a bit erratic but you can kill them as melee with some skill. Just poke carefully. Once again crystal sword makes this much easier.
You shouldn't attempt to fight leviathans til your speed is high/maxed. I've killed them plenty of times with melee before, not even with a crystal sword, but you need to move fast when they start spraying bullets everywhere to avoid getting shotgunned. I generally don't like fighting these.
Not giving a tactic for genies or flying brains because both are fairly easy. Melee classes eat flying brains right up regardless of their damage, since defense makes them hit so low.
Geared melee is most loved in the Abyss of Demons dungeon. Countless casters get insta-munched by brutes when they try to take the lead, while a knight or paladin can handle the initial surprise. I realize alot of these "tactics" resort to "raise your speed" and "get a crystal sword" or "get close and circle" but that's just how it is. Snake pits, and it's boss, Stheno, is incredibly easy for melee, so go take out ogre kings or lizard gods and the like to spawn pits and clear em to get your speed up. All you have to do for snake pit is break into a chamber, dash into it, and start rounding up the snakes like a sheep dog and poke them down. For Stheno, you just poke her during her first phase, back off for her second, poke her into her transition into her third, back off, poke her into her transition back into her first (and during) and repeat.