Any tips for Base Defense?

I've been playing for roughly a week and have made great progress but in base defense the furthest o get is about wave 26/27. It feels like the difficulty jumps massively after wave 20. I merge my units, level them up, but even a seemingly strong team struggles and often loses.

For those who have gone further, what's the secret sauce? Is it unit selection to start?

Any tips/thoughts are appreciated thank you