I don’t think it’s actually that one sided for Medea, tentacle material wish fulfillment aside.
Medea’s summons are garbage, seen from UBW, those skeletons didn’t do anything even to Shirou/Rin while Gilles’ even gave Saber with a proper Master trouble.
I’d say it would take more mana for Medea to blast the things than for Gilles to create.
Another psychological tactic is that Medea, from UBW, frees the captive children from her original Master, and probably has her own traumas regarding them (while the myth is famous for having her killing her own children, there are others where she doesn’t and others did).
Gilles has no problem killing children, which might trigger Medea to a reckless action into a trap. Also the part where he sends a child to Saber and the kid blows up, could be a nice bait.
Medea isn’t as a cruel and pitiless as she likes to be seen. She doesn’t kill anyone from mana drain in UBW, despite her Master not giving a shit had she killed the entire city. She let’s Taiga go unharmed. Doesn’t betray or mind controls Soichirou either.
If Medea found out Gilles’ hideout in Zero there’s no way that wouldn’t cause a massive trauma trigger on her, something Gilles would be able to exploit (despite his insanity, he’s quite intelligent and tactiful, considering how he went for Saber, planned his hideout and etc).
I don’t think Gilles will be able to summon his C’thulhu wannabe in most scenarios. It seems to take forever and consider a giant amount of mana and him standing there for hours, which Medea would notice and would retaliate.
Unless she gets wounded in one of those traps/triggers mentioned above and Gilles uses the time, then it’s game for Medea. Or Gilles baits Medea with children on the other side of the city while he does the ritual.
She’s from the Age of Gods and a disciple of Circe, but she’s more a healer/diviner/adviser than a “fireball a mountain away” type of mage. Rule Breaker is never touching Gilles either.
The question would be how Rule Breaker would interact with the C’thulhu…