Thanks for the feedback. Maybe I did go to far with the balancing and it's too balanced now, making a back and forth inevitable.
The opponent will go through phases of saving up mana and spending all their mana. When you damage his base he will immediately spend some of that mana to push back on your attack. So maybe you hit him in the middle of him saving up on mana which is why he was able to spawn so many slimes. But keep in mind that every slime you kill will also grant you a lot of mana to use on your own attack. So its not really a bad thing.
You can also try lowering the difficulty level or starting a new round where the AI will behave differently.