Just finished a session where all 10 participants earned the "It Burns" cheevo. Here's what we did:
Set up two platoons of 5. EVERYONE select the LeichtTractor with the Leighttractor III package and a fire extinguisher. No special shells or crew skills are necessary. Get both platoons in a Tier I 7 vs 7 battle. Call out to your party members where the non-boosting, random players are (there will be two per side) and once they are dead, meet in the middle.
*Edit: Make sure the fire extinguisher is a MANUAL extinguisher and not an automatic fire extinguisher. The Automatics have an active boost that will decrease the tank's chance of catching fire by 10%.
One of our fellow boosters (forgive me, I can't remember which player first pointed it out, but he is awesome!) told us the sweet spot to shoot on the LTractor to set it on fire: DO NOT shoot at the front panel. Look at the SIDES of the tank above the tracks toward the front. There is a little headlight, and right behind the headlight is a small round circle. This circle is the EXACT spot you want to aim for.
Get close enough so your shots won't stray, and use a light tap on the trigger to fire a single burst. After a couple shots the target tank should hear a "fuel hit" notification and the next couple shots should set the tank on fire. Then use the fire extinguisher to put it out quick. Don't just blast away or the shots will stray or damage it too quickly. Short, controlled bursts are the key.
Note: Some people say shoot the left side, but my experience was that the RIGHT side of the tank seemed to consistently yield the fuel leak and then fire results. For all I know the effects are exactly the same on either side of the tank, but it seemed to me like the shots on the right worked better than the left.
If each platoon has their tanks sit still (once all the "random" players are dead) the person going for the cheevo should have no problem getting two fire results shooting at only 3 tanks, and if they have fire extinguishers they don't even need to die. Then someone in the opposing platoon can take a turn setting your tanks on fire. At least two players can get this each match, one from each side. If you're lucky maybe more. Then you can finish off the match with a base cap, some double ram kills on the weakened tanks, or whatever.