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25 G

You have to sneak around the room (avoiding Freeze's line of site) and execute "sneak attacks." No one attack can be used more than once because Freeze will disable them, so variety is key. I'm not sure if it scales to difficulty, but on Normal it takes 5 hits. The attacks are:

1. Silent Takedown (from behind with Y)
2. Glide Kick/Drop Attack (Wire walk, or simply Glide Kick him from the second floor)
4. Electrocharge The Magnet(s) (when he is near a magnet shoot it with an electropulse RB)
5. Takedown from inside the floor Grate
6. Electrocution (there is a pool of water on the lowest level, a button inside the door right next to it electofies it.
7. Disabler (used against Penguin when he has the Freeze Gun)

Allegedly through-the-wall takedowns also work, but I couldn't get the on-screen prompt. If anyone can confirm/deny I will update.

Below is a video of all but the disabler courtesy of Gamespot.