Immortal extradimensional being with telepathic (possibly mind control) & shape shifting abilities vs super soldier with an eye patch.
Not to downplay Deathstroke’s feats because they are impressive, but Pennywise is something beyond Deathstroke. Coming from the Macroverse, existing since presumably the beginning of time; Pennywise is nearly impossible to beat. The only reason the Losers Club was able to hurt him was that they were kids & had The Turtle on their side & became able to face their fear of him. Even when they came back as adults & supposedly killed him, it’s written in the Dreamcatcher novel, “Pennywise Lives”. So IDK, maybe Deathstroke could find a way to hurt him, but Pennywise eats him in the end, starting with his other eye.