Yeah haters can be a big nuisance in the early game. The thing to remember is that haters max out based on the highest grade fighter you have available to you. So if you have a grade 2 figher, the level cap for that is 50 so running into haters in their 40s and what not will soon become commonplace.
My rule of thumb is to never over extend myself as far as moving through the tower. My number one goal when moving through the tower is to find the next floor with an elevator, activate that elevator, go back to the waiting room, level up and gear up and then head back out. This will allow you to level yourself at a pretty good pace and minimize the level gap you'll get with invading haters
Weapons can't be broken down. If you have something in your inventory that you don't like, just toss it or hold on to it to sell it back at the shop in the waiting room. I believe once you pass floor 6 you can access different types of fighters besides all rounders but after you beat floor 10 is when you unlock the next grade/tier of fighters that have higher stat/level caps