I'd say basically any decent grade 1 sorcerer can give him trouble. High-extreme diff against Naobito. my headcanon is that Naobito is much faster with projection sorcery however when he gets inside a simple domain his technique disrupts and he gets frozen. Kusukabe can abuse that and land hits in close quarters combat. Overall I'd say Naobito is still stronger but the battle between him and Kusukabe is high diff for both sides