Best way to avoid this bug is Beat Nishi with a Waterbug, like you all should be doing
A good tactic if you just want a quick win, but for power levelling lower pets try using a Water Waveling. As long as you fight Siren first (which will be the majority of the time) he will just heal himself and occasionally cast sunlight. You can therefore just spam tidal wave until his back line pets eventually die, then finish him off with water jet. This allows you to power level almost any lowbie pet. As long as they can survive a couple of sunlights, you can just keep them back and swap them in for a single round so they get the credit for the fight. It's rather good xp on a low level; with safari hat a level 3-4 pet will ding to 12-13 from this fight alone. Critters are more survivable because of their resistance to sunlight, with the opposite being true for mechs. If you need a bit of extra survivability for the lowbie, try using a pet with a group heal in the third slot, such as a sheep or cricket.
This isn't my tip btw, I read it somewhere (probably on this site). Just thought I would add it here as I've found it a very handy way of levelling up pets.