Build monsters with a damage expectation of 3x Standard monster damage, increasing to 4x when the monster is bloodied.
ALL solo monsters should have at least two standard actions per round, and two immediate actions per round. Factor any damage from standard actions into the above model.
Solo monsters should have a "Cleanser" ability usable once or more per encounter, whose effect is to effectively make the monster into a different creature altogether as it pertains to status effects. This should also apply to the monster's own abilities-any temporary buffs should also be negated.
Utilize a Save at Start of Turn effect for End of Next Turn type effects.
Solo monsters should have a limited means of negating marks.