There is an option to use the mouse and a lot of people seem to like this, but you should use the keyboard if you really want to get good. You can flip around to shoot a monsters behind you more quickly. (A joystick or gamepad is even better.) You can move at diagonals, so make use of this. It will cut your travel time in half.
Time is your enemy so don't waste it, but sometimes it pays to be patient and wait in a good position to shoot the monsters. Yes your health is always ticking down, but it will go down faster if you are touch a creature.
Buggles: these creatures can be difficult to shoot if they are adjacent to you because they move around so much. It is best to shoot them from a distance, along a wall or from right behind a corner is best. If you find yourself close to Buggles it is best to walk away and them flip around and shoot quickly.
Bats: pesky creatures; they don't do much damage, but they are annoying. Shoot them from a distance. Timing is everything.
Poolball: this thing can't be destroyed, so don't waste your time trying. You can best avoid the 8 ball by running across the hall right after it passes.
Beholder: a powerful creature, but it can be destroyed with several direct hits. It is rumoured that the beholders have magical abilities and can summon other creatures.
