This is quite straight-forward. All Tier 1 tanks are light tanks, and a lot of Tier 2 tanks are Light tanks as well! Light tanks have a solid diamond by their Tier number.
Just start a game with your light tank, if you get destroyed you can just go straight to your garage and pick another ready light tank and play another match! It will take some time to get the last Light tank matches played achievement but don't spend too much time watching games finish after you're out and it should go faster: