Some easy tips on how to get to 100+ frog kills with little effort. Most important is staying active on selling and rebuying to keep that interest rate going.
1. Same/All Random required.
2. Selling all towers just before interest.
3. Always using your best tower.
I know this doesn't say much but let's go through an example.
First element is Nature.
Nature doesn't do AOE so nature isn't an alternative.
We go with cannon towers. start with 5 cannon towers on the top left 2x5 area.
Sell all after 14seconds. And rebuild in the middle if any still alive.
Repeat this until level 4.
Level 3 Fire can sometimes be a hassle since they run quite fast and take a hit to kill on Hard+.
Keep selling, building cannon towers untill level 12. At this point you have two elements to chose from so you aren't weak against any.
Keep away from single Fire towers. Stay with strong towers like Nature/Darkness/Light. Also Water is very good untill aprox 27-29.
At higher levels keep to the 3-combo towers. Duals even level 2 don't do much damage. Except Well and Forge. those are essential.
Since we are talking random I can't say exact towers.
I know this doesn't say a lot but with these easy steps I have reached frog rank 62 and classic 220.
Interest rate and getting that gold is the most important.