i think this game is not so hard, take multiple arrows and shuffle, after the first time skill cool down, you may have many combos, and stay alive as long as you can, while you level up your HP will recover and also gain some extra HP. i think this is quite fun, i play it for 4 hours hehe
okay, here's my tips to pass last stage, I put Archers to the right three rows and put knights in front of them, and then I put three rows of freeze trap and one row of spike. I use Cannon-Shotgun-Arrows combo without using any spells and pass easily. Upgrade Champion tree to max, freeze trap (maxed), knight (maxed), spike (maxed), money (maxed) and also berzerker. Pass the final stage you only need Lv. 19