A wave is a group of enemies that arrives close together. Watch where the group starts getting through, since that part of your defense may need help.
In Bloons TD Battles 2. You can send groups of Bloons to your opponent, giving their towers more targets to handle.
03Kingdom Rush
An upgrade improves a tower you already placed. If enemies slip through a stretch no tower can reach, building there may help more than improving a tower elsewhere.
In Kingdom Rush. Towers can be upgraded and specialized as you defend stages in the campaign.
04Tower Rush
Head-to-head means playing directly against another player. Watch your own defense and what your opponent changes, because their next move can change where you need to focus.
In Tower Rush. Players face each other in real time on the same battlefield.