Soloing bosses or dungeons that should probably be done with a team.
Teaming up on difficult content with menu chatters I can't communicate with....and always persisting until we succeed no matter how many mistakes are made or how messy it gets Darkmoor Graveyard pre-nerf with menu chatters was very long but soooo much fun!
Helping people who are struggling and letting them call the shots even when it might not be the best strategy. I always let them know the best method I know of, but if they want to do it 'their way' I'm their helper minion in that scenario and I support them the way they want me to. I confess I do put in some 'secret' spells for when it goes wrong (eg extra heals, feints, reshuffle etc), but I never 'steal their kill'.
it is actually a challenge to do sidequests in the game. It disadvantages you if you do sidequests, because you can be level 70 in celestia, and you will only level up twice in the whole world. this would make it painfully slow. I don't do ANY sidequests in the entire game, except for the ones that give me spells and stuff. I have created every school wizard in the game, and have not done a single non essential side quest. But, different wizards have different opinions!
Darkmoor Graveyard pre-nerf with menu chatters was very long but soooo much fun! 
Thomas Wildstaff, 93
