I expect that hydra arrow just takes initial hit dmg and scalar with that. Nothing more.
Using it on solara and frog would be a waste of inventory slots then.
I have no hard facts for that but this would be the straightforward solution. Also it would have the same dynamics as these towers have which is what you would expect here.
It’s great in end game (if you got a lot of stats and multicrit from potions so that items get you less than 40% bonus.
Maybe not on solara or frog as mentioned above.