Easy. The King. He’s one of the best blue 5* for red titans. As an elemental debuffer he even works better than Frida in this scenario.
As mentioned above, I would also consider Aegir, at least take him to 3/70. He’ll make sure your heroes won’t go down easily.
If you’ll end up with a line up somewhat like this: Athena - King Arthur - Aegir - Grimm - Wu Kong
… The red titans will cry.