Have a level 9 damage spell like shatter, tornado, fire storm, or glaciate. Also have a good axe and high axes skill. Resistances, AC, EV etc. Use Makhleb. Have controlled blink. Good luck.

Another thing to consider is that online allows you to avoid the temptation of save-scumming, if that's something you ever find yourself tempted to do. It forces you to play honestly.