If you get a creature with only one body part (blob), then metal, especially steel, creatures are functionally impossible to defeat in melee. You can't cleave them in half because there's no second part to bisect, and it's basically impossible to chip steel, even with adamantine. These are, fortunately, rare. Try lowering the number of clown varieties to improve your odds.
Even without that, syndromes WILL kill your whole melee team. Water just stops the spread of syndromes, not the infection every dwarf in melee range will get these syndromes. Since clown syndromes tend to be "everything gets necrosis in 1000 ticks", expect your whole melee team to melt into miasma halfway through the melee. Without this, I'd suggest spreading your enemies out, so you can take on small chunks at a time. With this, enh...
Also, some of the demons are very likely to be webbers. Webbers will web their own teammates frequently, but they'll make a mockery of even the most elite axedwarf. Webs are also AoE attacks, and if they come with a projectile spitter, not even marksdwarves behind fortifications stand a chance.
Basically, if you get a fleshy clown that "fights fair", you can easily take it one-on-one, but enough of the clowns fight with AoEs and skill-voiding certain death or nigh-invincibility that unless you get extremely lucky (or dramatically mitigate clown variety and just get sort of lucky), then a head-on charge is almost certain to fail. Even throwing more dwarves at the problem is just going to result in more death to syndromes.
Your best bet to fight a little smarter, not more Stalinesque is to have a huge network of drawbridge-gated sub-arenas with bait animals that split off into multiple doorways to other sub-arenas, so as to hopefully split the clown horde off. (Possibly having some bait animals behind foritifications so that they don't attract melee clowns.) If you can get the syndrome, exploder, and webber clowns completely out of an arena, you can then send your melee squad in for a "fair fight". (So long as there are no solid steel clowns or bronze blobs or something...) When the time comes to fight the exploders, dump armed goblin prisoners on them to "remote-detonate" the bombs. Webbers need to be fought preferably away from the projectile spitters, but if you can't, I'd suggest dumping all but a breeding pair of your fortress's livestock on top of them to create hundreds of bait creatures for repeated webbing and decapitation while your marksdwarves can hopefully whittle down the enemy numbers. (Armok help you clearly hates you and laughs at your cries for mercy if you get iron webbers or something.)
In all, though, it's more actually possible than it used to be in .31, when I think they were 10 times larger, and could be made of slade, fired projectiles much more rapidly, and had cave-in dust as an attack.