For the bottom player, I couldn't reach the teleport in one day, twice. It feels unusual to see hordes of level 1 monsters in the final stages. I can't say anything specific about the experience gained by joining monsters yet. I need to try it more. I can make a few suggestions:
1. Either make the movement buffs 5 times more effective, or place them more frequently on the path to the portal.
2. There are many magic eyes, and not all of them are placed in good locations. It would be better to replace them with blocked pioneer units under the player's flag, or place flagged objects there for better visibility.
3. To make all monsters visible at once, you can flag the middle level 4 monsters; that should be enough.
4. Joining monsters behave differently. Some run to join the army on their own, which is very nice but a little unsettling. Some stay in place as usual. Maybe make it consistent for all?
5. There are places, for example, for the bottom player in the bottom row of monsters, where when you buy level 3 (or another) monsters, after opening, it turns out that you can attack neighboring monsters. This could lead to the fact that it won't be possible to add monsters to the army without a fight.
The new arena looks quite playable. A small detail, in terms of design - for aesthetics, you can embellish the landscape.