This was taken straight from my TA walkthrough
-Run around the courtyard and locate the giant that is tied down by some telemarines (he's beside the staircase where you went to do the level, Protect the Battlements). Kill the telemarines, and hop onto his back (as minotaur). Now, walk over to the main gate, and keep button mashing "A" to close it. After the cutscene, turn around, and head towards the gate where you got Susan's horn. Kill the 2 knight telemarines there while on the giant (I just spammed "X"). After their demise, destroy 30 more telemarines, which leads you to more cutscenes, and the achievement