I got to lv 42 by making it a 2 player, keep only 1 entrance open, and keep constantly throwing grenades. When one player is almost out, try to get more while the other player keeps throwing. Might take a while, but it works.
Easy Card, i made hardest difficulty with all the things checked for higher multiplier, then just walked around exploring instead of fighting, and used the scrolls that make you pass 1 enemy to also not fight. Pretty easy.