When to face Diablo?

[Bob the Beholder]

Really?  Even considering all the problems I had against the other act bosses?

Well, the problem here is that most of Diablo's attacks are either weak, or easy to dodge.

Andariel, however, runs around like crazy, and does heavy poison damage. Duriel slows you down and moves fast, so his heavy damage attacks are hard to avoid. Mephisto's attacks, also, can be pretty hard to dodge. But Diablo's heavy damage attacks are mainly the lightning breath, which you won't get hit by if you react fast enough, and the firsestorm attack, which isn't too hard to sidestep.

His melee attack isn't too hard to deal with, if you've got decent life, and his fire nova is rarely worth noticing, if you've got a decent amount of fire restistance.

Granted, you will screw up and get hit occasionally, but, unless you have such pathetic life and resists that you get insta-killed, the lots of potions I suggested to bring can keep you going.

If you haven't fought Diablo, or haven't in a long time, then I'd be careful about it until you can get his attack patterns down, and can successfully dodge.