Ah, a friend told me to leave that boss until later since he's meant to be really hard so I went and did the Dead Dunes quest instead. I'm on the last few hours of Day 4 at the moment and I'm just doing a few sidequests in Luxerion before Day 5 begins. I'll finish off the temple on Day 5 and then do the last Main Mission on Day 6 I think.
The item you get for killing them appears on the ground after the fight so be sure to check before you move on. Also be careful about driving too many monsters to extinction, you do need them for EP.
I've just found out that you can drive monsters to extinction, you end up fighting an empowered final creature and after you kill it you can pick up a rare accessory or weapon. It's a pretty cool mechanic.