Check back on fight between him and Ulti and Page One. first damage he got was off-screen, but you see clearly Ulti hitting him for second time and it cracked his skull, but even that didn't knock him out as we are shown him active even while down and active in next chapter when with Nami, Tama, and her dog guardian (forgot his name sorry). I mean Usopp's is underrated on his durability as let's not forget he took many damage and can still get back up to continue fighting not just in pre-timeskip, but post-timeskip too. It happen in same arc, not trolling, just saying.
Would you guys honestly say Kamazou over Ulti cause that wouldn't make sense in power-scalling especially he was nerfed. Damage skull is more painful than shoulder wound, yet Usopp somehow stays awake, but Zoro soon fainted, so feel like Oda switching dynamic on who is durable in the crew more than others.