1.This is a touchy area that I'm going to tackle,but if I must...
Be careful with boss placement,don't put him too close to an entrance or exit...make the player sweat a little.
2.Also,when using multiple bosses,make sure there is enough ammo & health to go between them.
3.An exception,here: if using multiples,you'll probably already have all keys necsessary to complete the level;then,maybe putting a random boss directly at the exit is OK.
