You can tank a dragon but you need incredibly high physical resistance to avoid all the captures and such. Oghren was a good candidate. I gave him a nice maul and a bunch of physical resistance runes/amulet/etc., and he stayed alive pretty well without a lot of potions. This was on normal, though. No way I'd try that on nightmare. I must have died 20+ times trying to kill the high dragon on Nightmare before eeking out a kill and that was with three mages trying their best to stun/paralyze it as much as possible.