In a recent tournament, I paired her with Grey Knight Interceptors through the Imperial Agents Daemonhunter Strike Force. Two Daemonhammers (one of which is for the Justicar since he has two attacks base) and three Swords. In this particular list I had Coteaz stringing along from the Castellans Formation. He isn't as fast but if you spread out your models, he can stay in contact.
St. Celestine and Friends tank the wounds as they get into assault range. GK Interceptors activate Hammerhand (+2S) while Coteaz activates Force if needed. Genevieve takes any challenges to keep St. Celestine and the Justicar free. St. Celestine whipes out some models at her initiative step, most likely before anyone else swings. The Swords and Friends get their wacks in at I4, most likely reaping some more models thanks to S5-6 AP3 while taking some hits which could be tanked by Elanor and St. C. Then the Hammers do the clean up at I1.
I experimented quite a bit that tournament, much to the detriment of my tournament score but to the benefit of my understanding of my army. St. Celestine can hang in there quite well. Her Geminae tend to die a lot since they double out pretty easy but they come back very quickly too.
One of my experiments was comboing St. Celestine and Friends with Assault Marines armed with Eviscerators. This proved less successful. Without Hammerhand, St. C & Co more annoying than dangerous while the general lack of AP3 amongst the AM's made them much less so. I can only imagine that Seraphims would be even worse.