You need one weapon that cracks the players up! Like, exploding clown heads. It needs to be somewhat powerfull to, like a gun that shoots a rabid weasle at an enemy, instinty destroying him! here''s the story:
your walking down the hall and you see an enemy.
enemy: hey a bogey!
you load your weasle shooter.
the enemy fires at you while your strafing.
the enemy has to load.
a weasle shoots out of the cannon and laches on to the enemy.
enemy (screaming): argggggggggggggh! no don''t bit meeeeeee! ahhhhh!
and then there''s silence.
you cock the weasle gun and continue walking with a smirk on your face.
"blow up a lawnmower plant, save a lawn gnome!"
You could be amusingly cliche and use a pen. (The pen is mightier than the sword.) A pen could have really great special attacks in the form of words that get written, then circle around the monster. Like, "Blah blah blah blah..." could bore the monster to sleep. or "Calculus!" could make the monster''s head explode .
Hey, what about equipping a useless weapon? Like a a Salad Shooter? Just imagine flattening leaf after leaf of crisp lettuce on that demon''s chest shortly before he gnaws you into a shredded rag of flesh. Yummy.