Thank you both, these were just the tips I needed. By adding Brave and Patience talismans to the team I not only completed the Insanity Assassin challenge but was able to auto every run.
Full Assassin team was:
Max. Eternity Eddie (Archer, Burning, 3*Cosmic Past)
Max. Lilith Nightmoon (Justice, 5*Cosmic Alchemy)
Max. Mitsuko (Brave)
Sevastapol [Max. Assassin War Golem] (Prudence, 3*T2 Cosmic High Virtue - Patience + 1x random)
Fastest run was 2m45 but I suspect that swapping out Mitsuko for Musashi with Brave tali's would be a little faster as there were quite a few times where I was waiting for enemy defence effects to time out before completing the stage. Didn't try it though as I'd already spent ages failing and was just happy that I was finally progressing.