He has hp dark I think.
It's a problem because they are usually what end up switching in on my Weezing to prevent the burn or resist an expected poison stab. I can usually handle a steel type by burning and using rest. But if I run into any with SD, it usually ends up being useless.
Edit: maybe I can use thief to steal their item after I use the chesto berry, but may end up getting a choice item or something
I'd recommend sticking with Assurance. Better to not take a chance, and they both do the same amount of damage.