If you don't want to wait, invisibility and a bit of luck will get you into the Titan's stronghold, but I can't remember if there is any safe place where you can recast it if things go wrong. I think you may be able to recast it in the chamber where you find the bow, but I'm not sure.

Killing the dragon is a matter of hit and move. The general plan is to run round the dragon in real-time until it tries to blast you and misses (hopefully!), then hit Return, attack it with your own ranged attacks (good wands are useful) in turn-based using only your fastest characters, and then go back into real-time and resume the circling. It takes a certain amount of digital dexterity; some can do it and others can't. A good plan would be to save your game after every successful attack, and reload if you get more than slightly damaged.

HTH!