Dungeons 3

This is done in the "A Disturbance In The Force" level. During the sacrifice of each Titan, Thalya is left frozen and must be defended against spawning golems. Thankfully these golems are quite weak by the time you have researched Titans and have spawned them. Healing Nagas also help to keep her health above 75% while she's locked into the sacrifice. Once the level is won and you've kept Thalya's health above 75% the trophy will pop.