Flash bombs can only be used once per level, and remember you only have 3 available for the whole game. Use them sparingly, preferably only in a life-threatening situation.
You can sit in a safe place close to a monster generator and simply pick the monsters off as they appear. Most monsters follow a set pattern of movement so it is quite easy to predict which way they will turn and wait in a place where it is easy to kill them as they appear in front or behind you. Remember to watch the timer though!
The lanterns and rings are quite often a deadly diversion as they are sometimes placed in dead end vertical passages. Assess the risks carefully before collecting them.
Be careful when entering vertical tunnels as you cannot shoot monsters on the other side until they are level with you in a horizontal direction.
A carefully timed shot can kill a monster in a vertical tunnel, but only when fired exactly when the monster moves into the square in which you are stood.