Yeah unfortunately he did count.
The wording of the achievement was "Beat the game without anyone leveling up (Includes GFs and temp party members)."
In all fairness it meant I needed to beat a lot of fish on Balamb beach (by carding them so I don't get EXP, only AP) and then play a lot of cards and modding the good ones. Then I used Break on as many forced (non-boss) encounters as I could
Well done to azuarc for finishing 67 places above me in the 2020 GOTD Guru Contest!