The_Fan said:
I don't care about reality, as long as I can picture it in a dramatic way I'm fine with it.
Hrm...
Let's say the archon is advancing towards a white dragon, weapon in hand, ready to strike, and the beast breathes a blast of swirling, blue-white vapors, with a gale-force wind...its movements slow against the wind. Ice forms on it...it tries to press forward, but the ice cracks...and the cracks go all the way through. As its legs give out it falls to the ground like the T-1000 and shatters.
Or the Ice Archon is suddenly encased in frozen air, unable to move...
Since he isn't dead yet, the ice will eventually break, but this weakens him - until the point where he just hasn't the strength anymore, and, after a few cone of colds, he is just frozen in place, forever.
Alternatively (what works for both Fire and Ice Archons): They are just swept away - their body combines with the "element" and the magical bond that keeps the Archon together (and moving) is broken.
On a physical/scientific level, all temperature is movement (usually on a very small scale, like from molecules and atoms).
A Fire Elemental/Archon has a finite temperature, which means there is a higher temperature. To be able to move in the way a Elemental does (he doesn't need a "fuel line" to move), something must keep him together. If something manages to "add" heat to the elementals temperature it is possible that this bonding force is overwhelmed, ripping the Elemental apart. The rips may be filled with other fire, but this fire is not part of the Elemental, and it works as well as cutting a human in pieces and trying to "repair" him by putting flesh in the resulting holes (which means it can work, but not on a "big" scale.
A Ice Elemental/Archon has a low temperature. But if he was at 0 K, he couldn't move (especially not in the way he probably does. Quantum Effects might still allow a little bit of movement, since he can't stay at 0 K anyway.). There is force at work that keeps him cold, but also not too cold. If you can get him cold enough, the force that stabilizes his temperature can fail. It's as if you would cut off the blood supply of living cells. If done shortly enough, it can be repaired, but if it's to long, the cells die. If enough cells die, the Ice Archon dies...
So, is that enough physics in your magical creatures?
Nevermind the point that you seem to be ignoring the fact it's an analogy and not meant to be taken entirely 1 on 1...We have 'Dessication' damage in Sandstorm (3rd ed) as is. As dessication is essentially the removal of water, you could, if you really wanted to, feasibly come up with something the opposite of this, forcibly adding water to someone's system. Both are, essentially, 'water' attacks. 
