For monsters and challenges, you'd be amazed what a party can handle when they all work together against a single opponent. A single fey creature like a goblin can be a suitable challenge for a group of 4+ players at that power level IF they learn how to use tactics, fate points, and maneuvers to their advantage. Alternatively, pure mortals or minor talents whose powers are not directly combat-related can be good. A ritualist, for example, who is working on summoning up a demon to terrorize the neighbor hood isn't that much of a combat threat if you catch him before the ritual is complete.