Okay, I finally managed to win, took me like ~6 tries i think. This game reminds me a lot about Tooth and Tale, which I love (as you can guess from the avatar xd).
My first approach to the fight was to buy only yellow critters and just kite the frog by attacking once, immediately retreat, reposition and repeat with the idea that their attack cooldown would run out when repositioning. It kinda worked, but I noticed that I can't switch my commands quickly enough. When I pressed E to retreat, I would need to wait ~2s to be able to attack again. I'm not sure if it's intentional, but it made playing with this strategy quite frustrating, because sometimes I'd think I commanded the units to do one thing, but they didn't do it. In the end I won by just spamming red ones, they are really OP. The biggest factor is that frog's melee attacks are way weaker than distance ones, so it's better to keep units close and heal them, rather than trying to dodge and navigate around the map.
The frog's AoE is fricking off the charts, it can wipe your whole army in one attack if you time things badly and it was the top #1 reason when I was losing. Making projectiles go through no more than, say, 3 units would really help.
I really liked that boss telegraphed attacks in a very clear way, it's such an important detail and props to you for implementing all of the necessary indicators (marking where goo projectiles fall, etc.). I wish the boss had some kind of repositioning attack, e.g. when jumping it could land on the opposite side of the map for example, would make the fight more dynamic.
Also, the very simple thing that would imho improve the game is to make the fight harder with time by adjusting frog's time between attacks based on it's hp.
Generally speaking it's a really good jam game. Allows for enough depth to develop strategies, looks and plays good, gj.