Step 1: Start Varnhold's Lot
Step 2: Your MC must be a girl
Step 3: As you progress, be nice and romantic to Maeger Varn. It's much easier to woo him, than it was for Guardian of The Bloom. Much much easier
Step 4: complete the dlc after the kiss
Step 5: start a game and during or after Bloom, you'll get a letter from Varn. Visit him. Time passes. Save him. Time passes. He'll request that you go to Lostlarn Keep with him. He just shows up when you go. You get the trophy once you save the girl
(Hint: equipment you find during Varnhold's Lot, will be for sale ONE TIME by a random encounter merchant. This equipment seems to be randomized, and it seems to include equipment that you didn't even equip on your Varnhold Lot companions... so perhaps try throwing away any and all gear that you don't want to buy. Experiment with save scumming if it pleases you. The only item i wanted from the dlc was a tank ring, and some stat-boosting potion-like books. But alas, i didn't care enough to save scum).