Combos
When the monsters appear, wait momentarily to see if a better placed one appears so you can complete a combo. Combos are triggered when your next tap is inside the circle created by your previous attack. Get as many combos as possible with each wave to score higher. However, if you need to eliminate a monster on the other side of the screen, tap it and ignore combos for the moment.
Upgrades
Start by upgrading your Health two times then Damage once. After that, complete the first level of Masteries in any desired order. If you have any coins remaining, spend them in Health again. Do not worry about the Power upgrades until you have the Masteries upgraded.