Well, it's pretty much staying alive when battling Arianna in the 2nd form.
Viewing post in BoxxyQuest: The Gathering Storm comments
Well, firstly make sure everyone has the best available gear from the DeviantArt shop. Also (or alternatively), buy a bunch of "Very Root Indeed" items, and use them when Arianna changes to her 2nd form. They'll bypass her increased defense and do some pretty massive damage.
If you need more in-depth strategies, try asking BTD6_Maker in the Discord chat. He's the one writing the strategy guide, and honestly he knows even more about the workings of the game than I do at this point. :P