Since there is a major energy drawback to using this power, one route for maximum combat efficiency would be to focus on Precision and/or Set-up.
Instead of forming huge chunks of ice and hurling/dropping them on your target, what about forming spikes from walls/ceiling, and simply creating a slippery surface to slide your foe into their own deaths / major injury?
It doesn't even have to be that complex, freezing the eyes of your enemy is another option, as well as freezing the air right at the opening of their nose, and when they breath/speak, freeze that as well and suffocate the target.
Defensively, the concept is the same. If you know where your opponent's attacks are aiming for, instead of completely stopping that force, redirect it using angled ice walls or simply liquid in the air. A Martial Art Style like Aikido would work well with this train of thought.
The whole goal here is to not exert a lot of effort, which could kill the user, but rather deliberately use precise force strategically.
Another, non-exclusive thought, would be to set-up your fights in a way that plays to your magic/strengths. Fight near a large body of water or a cold environment that minimize the energy you need to consume to drop the temperature farther.
Tools, like a bottle of water, liquid nitrogen cartridges, Water-filled vest, water balloons are all examples of how a smart user of this magic could prepare themselves for combat before it even happens. If given enough time before a fight within a set location, setting simple water traps could be lethal with the right circumstances.
All of these answers are only pertaining to dropping the temperature of the surroundings, if pressure were an option like suggested, there are a slew of other principles that could be applied depending on the circumstances.