The character who I'm talking about is faced with certain death and has to freeze a nuclear exposition that's coming right at him, would this work, and how would it. This character has ice-based powers as well as superhuman strength. He has reached extreme sub-zero temperatures including almost absolute zero which he would have achieved but didn't want to kill someone. He has also taken extreme heat which he has frozen including instantly freezing several explosions that were smaller than a nuclear bomb but big enough to level a city.
A blast wave requires a medium to travel through
The explosion has happened. They are fast. Also bright, but that is done now so hopefully he was looking the other way. Now there is a shock wave coming.
Cooled gas shrinks. If he can cool down the gas in the path of the shock wave it will shrink and its pressure will decrease. If he can cool down the gas until the pressure is very low the shockwave will not be able to cross that area - just as soundwaves cannot propagate in a vacuum (and in space, no-one can hear you scream) shockwave needs a medium to propagate through. He can keep the shock wave from hitting him.
If he has god power he can produce a vacuum and the blast wave will have nowhere to go. If he cools down the entire area such that the gas condenses into liquid (or solid - snow!), it will blunt the blast everywhere. This is well trod ground on WB stack: nukes in space. In space nuclear explosions are only a flash, no shockwave and so nuclear explosions are much less destructive. Of course there will then be an implosion as the rest of the atmosphere rushes in but that is how it goes.
I am wondering now about a shockwave moving across a bubble of air and hitting vacuum. Where does that energy go? I think the wave might be reflected back into the air where it came from and heat it up again on the way through.