How do I melt the Frozen Riddler Trophies?
Solution 1:
Frozen trophies can be picked just by standing next to them and picking them up (rather than by the BatClaw from afar). If a frozen trophy is floating on the water, you'll need to throw a freeze grenade next to it and stand on the platform to pick it up. There's at least one instance where the trophy hangs above the water, in that case, you'll need to drop it into the water by cutting the rope it hangs from with the Batarang.
Solution 2:
You should be able to pick them up just like any other trophy, frozen or not.