For max effect (and this is no secret,by the way) you need to ring the cryo tank with explosives to heat it up as it is being released (a necklace?), then ignite it before it vaporizes and mixes with the atmosphere too much. For the most damage to occur, the flame needs to blow outward along an already expanding wave front of gas and not consume it before it reaches its max diameter.
I'n no expert, but, wouldn't a armor piercing incendiary say in the .50 cal range do the trick?