Tip to reach wave 300 on hard: Invest heavily in elemental piercing after you reach wave 100. This will nullify the threat of the universal enemies. Afterwards you'll essentially be blowing them back like a leafblower.
Tips for the hard badge: Always try to perfect tunnels on low multipliers. Try buzzing the tower as much as you can. Do a snake-like pattern for best speed at low multipliers.
Breath-taking graphics, amazing storyline, great soundtrack and many other upsides. The final boss made me shed a tear from how amazing this game is. Seeing it end was like seeing my child die. Thank you for this great game.