Boo! Trick-or-Eater is thankfully no Asmodeus despite what the preview image implies; instead, it’s a Soil Eater under another name.
For a Level 20 boss, Trick-or-Eater is actually really easy. All it does is Brave to use Earthquake to damage your party and perform a regular attack, before becoming completely helpless for one turn.
So all you really need to do is Default on the turn it Braves, unleash your attacks on the next turn (and heal if necessary) and then rinse and repeat. Try to take advantage of its Water weakness if you can.
The purpose of this Nemesis is to suggest a particular date: observe the pg, EXP and JP after defeating it.