Persona 3 Portable

This achievement is unlocked by defeating the second main boss in the storyline, taking place at the Dark Hour on 6/8. They're named aptly for the respective arcanas that are a hallmark of the Persona series.

In terms of difficulty, this fight is actually relatively easy, and depending on the level of your MC and strength of your Personas (especially for strength and magic stats) it's possible for you to beat this pair without them even getting a chance of attacking or casting a spell. (Thar's how I beat this pair and unlocked this achievement, I grinded up with FeMC to level 29 and fused a lot of Personas like crazy. I wiped the floor with Take-Mikazuchi at level 27 with strength and magic stats at 23.)

The pair differ from each other in regards to types of attacks that they hold weaknesses to as well as completely neutralize. For Emperor, he's weak to magic attacks while impervious to physical ones, and vice versa for Empress. This sets well for an easy One More and follow-up All Out attack opportunity for you and your two party members. If your level and Persona stats are absurdly high, you can finish this fight as early as your second party member's turn after MC/FeMC's. But if you're going into this fight at lower levels, and/or playing on higher difficulties, you're gonna need a few of these All-Out cycles before you can finish them off. One thing to look out for with regards to any defensive shift by the bosses is the Paradigm Shift, which shakes up the weakness and resistance affinities of the bosses, making the battle under normal circumstances a matter of trial and error. At this stage you should have at least one Persona with an all-foe elemental attack spell that you can use to work around this changeup. (Elemental attack gems you've collected from treasure chests are worth considering using in a situation like this too.)

The achievement should pop-up on screen when you get through the battle results and go through the aftermath cutscene.