Tower tactics.

It’s better to start at 200% difficulty, as others have said, by choosing Aine (after all, 350 x 6 = 2100 additional gold in the first week at 200% can make a big difference). For leveling up, it’s best to choose someone from the purchasable heroes; the best candidates are experts in armor, logistics, or offense. If none of these are available, beastmasters or some mages (Grindan, for example, quickly gains mass slow, although his special ability isn’t the best at 350, but he breaks through everything quite easily, which allows him to level up quickly and can start clearing out resource mines earlier).