This isn't very big, but I suppose it's still worth posting.
The Deep Freeze rune for Frost Nova says, "If Frost Nova hits at least 5 targets, you gain a 15% bonus to Critical Hit change for 12 seconds." However, it doesn't specify that those targets have to be enemies. Hitting objects will count towards this. If that group of 3 champions are giving you a hard time, get them next to a couple of barrels and freeze them for 15% more crit.