To unlock 'Not My Type After All' you must defeat Boudica during the final chapter, The Son of Rome.
It's a fairly easy fight, with the main tip being :
Don't attack, just wait to counter hers.
She starts off with fairly basic three tiered attack. It will either require three fast deflects, or two fast deflects with a delayed one. Just watch her, it depends if she strikes fast, or spins and strikes.
Whenever you block her 'final' attack, charge X and land a heavy sword thrust quickly followed by two quick thrusts. This will be your entire attack pattern.
The rest of the fight is just about blocking - gradually her attacks get longer, I think in the end you are blocking five times before she will stagger.
Once she's defeated, the achievement will unlock. Story related and can't be missed.