Don't think so.
I beat the game with every character almost first try.
It depends on which moves you choose,
and how much your combo is.
I usually use the scream fully upgraded, where you get armies of people and then after I rack up 50 at the boss level, they beat him up pretty easily.
Also,
with a high combo and a machine gun, I killed the boss in under a minute.
The ammo boxes are not low, it depends how much you use the ammo. If you use a machine gun and only have the machine gun, obviously it will run out quickly.
This game has a little RNG, but its mostly skill and choices.
In conclusion, you're just bad.
(also, for the gravity thing hide behind the enemies. Or if you have an army he will shoot your goons instead of you.)