BUG I attacked germany with 0 troops and all my stats returned to the original at the start of the game. Except my economy was still at lvl 3 but my income was 2k
To reach wave 40 have a warrior, ranger, conjurer, cleric. Each character needs around 240 hp and buy all the soothing equipment you can; my characters had 2-3 + natures balm= 40+ hp regen per turn. Make sure to have skills that last the entire battle.