Mercenaries 2: World in Flames

Go into each faction HQ. and open fire. Let them call for back up. Once it's hostile, move on the the next faction.

All you need is 800k and have meet every faction... first turn your autosave off so it does not mess with any other progress.. Next go buy 5 artillery strikes 750k and 50k for 5 medevacs.. Now all you have to do is transit to each HQ throw an artillery strike on the HQ itself then as soon as that faction turns hostile medvac out. takes about 10 min

After you capture Solano you will be able to buy nukes from Eva for $1 million. Buy 5 of them and nuke the headquarters of each fraction. Blowing up the headquarters will make the fraction hostile instantly. Once you get it don't save, the only safe way to make the fraction friendly again is to bribe them. For a quicker way, join a game with a friend who already has ticked off everyone and you will get it instantly.

*ALERT! POTENTIAL SPOILERS!*

Really easy to get later on, especially starting with the autosave you get after you've beaten the game. If you completed "The Price of Gas" for China (including blowing up UP HQ) you probably already have UP hostile and either the Allied Nations or China hostile, depending on which side you chose for the Battle of Caracas. This leaves just the Pirates, P.L.A.V. and either China or the Allied Nations. If you want a faction to instantly hate you with a couple of button presses, drop a big airstrike on their HQ building (technically, any building you can enter works, but if you're going to piss them off anyway, why not destroy their HQ, eh?). BOOM! Instant hostility. I just flew a chopper to each of the factions' HQ's I needed to make hostile and dropped a MOAB, bunker buster, or some other large ordnance and had a lot of fun doing it. Afterwards, you can just quit and reload your save where only a couple factions are hostile.