Waves are not a steady climb. They spike, then ease, then spike harder.
The shape
Flat waves sit between spikes. Use the flat ones to bank gold and build coverage. The spike is where a thin board breaks, so build before it, not during.
Read the wave
Each wave throws a mix: more enemies, tougher ones, or a new lane. The wave database lists what each sends, so you can see the spike and cover it first.
The spike
Pressure spikes before bosses and at set marks. A board that clears flat waves can still break on a spike if one lane is thin. Deepen the weak lane before the mark.
The gap
Most wave deaths are a lane you missed for the enemy type that wave sent. Cover the type, not just the count. A gap ends the run regardless of your best unit.
The placement guide covers where to put units so they actually meet the wave.