I guess you are new to the game, right?
For non-melee characters, fighting Duriel involves a lot of running away, in particular in normal mode where he (or she... nobody knows) usually kills your mercenary in a few seconds.
Put just as much into strength and dexterity as needed to wear your gear, the rest goes into vitality. Early on, it could be useful for casters to invest a bit in energy, so you have a bigger mana pool.
Your level is OK, but you could level up a bit in the valley with the tomb entrances and/or the tombs themselves. If you aren't on hardcore, it might be an option to attack, die, come back, attack, die etc. Don't forget to put your gold into the shared stash (I think that even on normal difficulty, you lose gold from the personal stash when dying) and cast another town portal before dying again ;-)
As you are level 22, I strongly suggest to make the Stealth runeword (see
here for a list of good low-level runewords). Get a non-magical armor with 2 sockets and put a Tal and Eth rune into in (in that order). The runes Tal and Eth can be farmed from the Countess, a non-magical 2 socket armor can be bought from Charsi... leave town, without changing between acts, and come back in order to reset her inventory.
Also have a lot of potions with you.