MASSIVELY FRUSTRATING. If you understand that at first, then you might not get too upset at how random and luck based this feels.
Eventually though I was setting tanks on fire nearly every game, and then it was just a matter of time--I got the double!
Here are the best tips I have:
1. You WILL get better at setting tanks on fire!!
2. I used Tier 1 tanks, maps are much smaller, tanks are much closer, AND, you can shoot some very obvious engine weak spots!
3. In general, the tanks engine will be at the opposite end of the tank from the turret.
4. Shooting sideways through the tank is the best way to hit the engine then light it on fire. Generally, about 75% to the top of the closest wheel is a good height to shoot through. If you can't shoot through sideways, reposition, or just spray into the tank!
5. Auto-loaders, the machine-gun like guns worked the best for me. They fire many shots in a row and are good at not killing a tank just hitting it.
6. It is apparently possible to light the same tank on fire while it's already on fire. This never worked for me, but it's worth trying if you get the chance!
7. I fully geared up my tank that I was using going for this: +sight distance, +aiming speed, and +crew abilities. I also had a 100% trained crew working on Brothers in Arms skill. I have no idea how much this helped!
For Tier 1 tanks, especially the T1 and Traktor, hitting them in the front will very often set them in fire. Aim for about 1 foot from the very front. The other Tier 1's can be set on fire but it seemed more random.
Last thing--setting a tank on fire is apparently some sort of a random percentage around 15-20%. This means it won't always happen. However, with the auto-loader guns, you have many more shots and highly increase your chance to have it happen with one burst of fire.
I got this on a T1 and a Traktor I believe in my last match.. I spotted, then half-way across the map aimed for the front of their tanks and blasted away.
I'm including a video of the second kill for the achievement: