It's got something to do with the elemental wheel. When you meet the criteria and defeat the monster, it will disappear in a dark cloud of smoke with "spellbind" over it. After the battle you can make a pact with spellbound monsters, bu it's randomly selected between those monsters.
I wouldn't really bother with those Tower of Mana monsters. You don't have a lot of control over the wheel at that point and you'll run into stronger versions of those monsters anyway.
Tag - "No trolling on my watch!"








