Halo 3: ODST. Endure: Survive 4 Sets of Firefight on Heroic difficulty with 4 people..
Not only does this achievement take about 2 hours to do, It's fucking insane, what's worse is that if anyone Disconnects or has to leave, you are screwed and have to start all over again.
The best way to do this is when you know you will have a strong connection for a few hours, get some friends who know what they are doing, stay together and be sure you make the most of the power weapons such as the Spartan Laser and the Rocket Launcher by using them to take out Wraiths and Brute Chieftans.
The only bad thing is Skulls, Black Eye is probably the worst. You need to Melee to get Health Back.
There are 5 rounds in a wave, and 3 waves in a Set.
That's 60 waves. Yeah.
The other 2 Vidmasters that are a ***** to do are Deja vu and Annual.
Halo 3. Annual: Complete "Halo" on Legendary with the Iron skull on with 4 players in ghosts.
Halo 3: ODST. Deja vu: Complete Tsavo Highway on Legendary with the Iron skull on with 4 players without any of you getting into a Warthog or Scorpion tank.
The only real reward is Recon, which TBQH isn't worth it, most Vidmasters don't even give you gamerscore.
If Reach has Vidmasters, yeah, I'll do them, they were challenging and in some ways fun with friends but never again those 3, they were fucking ridiculous.