I'm Kadaeux, one of the sites premier nation RP GMs, though mine do have a bit more rules background than most here.
Depending on the GM it can happen several ways.
The first, and easiest, on both GM and player is if the players in question mutually agree how its going to go down, whats going to happen within the rules etc. In cases like these things are much easier.
The second, and more taxing on the GM, is the players in question asking the GM to determine roughly how it'll go down and they can RP it out between one another.
The third, and most taxing on the GM and players, but also the most fun, is for the involved players and the GM to get into a live chat of some kind and "roll it out" allowing luck of the dice to play its part.
However one should remember basics. If someones 10 tank divisions are up against a sniper in a watchtower with a 'sniper here' sign over it then, well, I should hope that the outcome would be fairly obvious (and messy).
Indeed, if you look at the threads I have done you can even find some examples of this
If there is anything more specific feel free to ask. But I do warn that my responses are coloured by my own Nation RP style and other GMs might have other rules.