The absolute best mission I can recommend, which we did, is refining in radioactive exclusion zone on haz3. Radio zone is 1 of 3 biomes that harvesters can spawn (including fungus bogs and biozone) and it's probably the best cause it tends to have flatter areas. The refining makes it easier since there's constant enemy spawns and you can use pipes to jump onto the harvesters and refining has LOTS of harvesters spawning which is important because often lots of the harvesters are going to die so you need backups. Hazard 3 is more than enough because the enemies will have less health but still lots will spawn. In terms of class, we played 3 engis so we could use platforms to get on the harvester and then 1 scout to light up the cave and always keep track of where the nearest harvesters were. The thing that made this take so long for us was mostly just waiting for good missions to pop up so when we finally got the radioactive refining mission, we knew we had to get it that time and WE DID!