This solution is for earning all "Won X Skirmish or Multiplayer matches" achievements.
This can take anywhere from 3-4 hours depending how fast you execute, my fastest match was 1 minute and 10 seconds, mixing that with setting the game up every time comes to about 2 minutes a game. But if you want all Skirmish related achievements you must also play all maps, play all modes, and play as all Leaders.
All Maps (18 maps):
All Leaders (6 games):
All Modes (5 modes):
After you obtain all those achievements follow this walkthrough for 1-2 minute wins:
*** Spoiler - click to reveal ***Your team: any UNSC
Mode: Deathmatch
Difficulty: Easy
A.I. Player: Captain Cutter
When you start the match fill your base que with Warthogs and set the rally point at the enemies base. Send your current warthog that you start with immediately and attack their warthog, attack with your warthogs until the base is destroyed, this can run anywhere from 1-2 minutes depending on map size and if you instantly take out their warthog.