I got frustrated trying to get Peccatum Originale so I made myself a visual aid and I figured was worth sharing in case anyone else has issues.
Let’s get right to it.
Step 1) Give everyone x2 jugs of water.
Step 2) Progress to the fight where, instead of gladiators, you go up against a team of mercenaries who are there to assassinate you on behalf of Mithridates.
Step 3) Have a party capable of surviving long enough to execute the plan (or remember to use restraint to keep the fight going long enough to do the same).
I quickly replicated the arena in Roll20 because I’m running a Lancer game there and already had a hex grid set up. The thick black lines are impassable terrain and the brown rectangles represent the barricades. If you’re having trouble matching the map to the arena, there’s an option to turn on visible grid lines in the game’s settings (they’re very faint though).
To get the achievement all you need to do is to throw a pot of water hitting everywhere you see the token with a little mech on it (an SSC Dusk Wing mech, if you’re familiar with Lancer). The blue auras represent the area covered by the water jugs.
You may get the achievement before completing the pattern because the arena starts with or retains water and scorch marks from previous battles, I’m not sure which.