Percy would probably be both terrified and intrigued. He's used to facing powerful monsters, but Cthulhu is on a whole different level. He might initially try to attack Cthulhu out of instinct, but quickly realize that his usual weapons and powers might not be enough. So he would likely retreat and try to come up with a more strategic plan.
Well, Percy is a very brave character. When he meets Cthulhu in fanfiction, he would first be in awe of the sheer size and otherworldly presence of Cthulhu. His first reaction might be to try and communicate, thinking that there could be some way to reason with this being. But as Cthulhu's madness - inducing powers start to affect him, he would realize that he needs to get away and find a way to protect his friends and the world from this new threat. He would then start to think about using the elements around him, like the water, in a more creative way to try and hold Cthulhu at bay. He might also seek the help of the gods, but he would know that they might be just as afraid of Cthulhu as he is.
Chapter 831, "Brother, who do you like the most?", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. It opens with Just as Jiang Shixu finished washing up and was getting ready to...
Chapter 648, "Even Wild Boars Make Way!", is the clearest early answer because it turns Pei Ye's situation into a concrete plot event. Early on, Jiang Li also casually picked up a tree branch and then directly.... By the end, This is just a program performance!] Jiang Li and Brother Pei....
Chapter 804, "Still causing trouble?", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event. It opens with He never imagined he would be scooped up like this! and closes on Upon seeing the indifferent expression in Jiang Li's eyes, he became....
A practical answer is Chapter 4, because it turns the idea into a concrete scene. The chapter opens with Qiao regretted her words as soon as they left her mouth., adds pressure through It's difficult for us when you ask for a guest's room number and key right..., and ends on A little girl acting.
Chapter 16 is the strongest chapter-level answer because it starts from Just as the words fell, Jiang Shixu swiftly cast his eyes downward. and follows through on Jiang Shixu panicked, set down the bowl, and reached forward to grab her....
Chapter 608, "Congratulations to Team Q for a Successful Escape!", is the clearest early answer because it turns Team's situation into a concrete plot event.
Chapter 858, "If You Dare, Take It", is the clearest early answer because it turns Mystical Event Investigation's situation into a concrete plot event. The chapter gives readers a real answer by opening on Changqing flicked his horsetail whisk, face wooden, and said,...
Chapter 930, "Everything Is Just Beginning [Grand Finale]", is the clearest early answer because it turns Hua University's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Yan was in a daze until he was invited on stage.
Chapter 800, "So It Was Her Causing the Trouble!", is the clearest early answer because it turns Meng Xuezhi's situation into a concrete plot event. A useful checkpoint is the way the scene starts with You, young lady, how can you be so disrespectful and interrupt when...
A practical answer is Chapter 9, because it turns the idea into a concrete scene. The chapter opens with Exactly, the Qiao Family isn't that great either., adds pressure through Since it was for the Jiang Family's welfare, then as the matriarch, she..., and ends on Qiao, unable to save face.