Can't remember where I read it, but at one point I saw a brief guide about which levels to farm for combination materials. However, the most advanced level it suggested was Return to Ruins. Since we have quite a few missions since then - including Desert and Eastern maps that drop units that always give a class bonus - I thought I'd ask.

Back when I was still on the desert missions I used Base Scramble II, and now I kind of drift around the various Eastern maps depending on what unit I'm trying to level (if I'm trying to level Spica up I'll go for Wind God Conquest for example), but which ones are the best if you're just farming the EXP bonus bronze units?