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5 G

Pretty simple.

LT + X: Quickfire gel
LT + Y: Quickfire Batclaw
LT + B: Quickfire REC (Remote Electrical Charge)
(LT)(LT)- *tap tap*: Quickfire Batarang
(RT)(RT)- *tap tap*: Quickfire Freeze Bomb

Freeze bombs should be the last gadget you unlock in story mode.

This does not all have to be done in one combo, but as the achievement says it has to be done in one fight (any mode works).

Once you get to the combat challenges, you will want to regularly perform all 5 quickfire gadget attacks every round to get the full 2000 point gadget bonus available.