Easy six levels of cleric. You will suck on the XP penalty for a while, but it's not that bad once you get used to it. I don't know what pantheon you use in your game world, but the domains I would go for when fighting undead are Life, Earth, Glory, and ; with Life or Repose as the top choice because it would give you Hide From Undead in your level 1 slot, which gives you the opportunity to use Grave Strike.
Put Grave Strike into all of your other slots. Your low Wisdom won't matter because you won't be casting any spells with saving throws, and you aren't getting so many levels of cleric that you will be missing out on bonus spells. By your 3rd level of cleric, pick up a Wisdom +2 item to bump you to 13 to get the bonus spells you missed, plus aid your Will save. Assuming you can pick up that Wis +2 item, you stick around in cleric until 6th level just before you would get 4th level spells to maximize your use of the cleric class and to eliminate the XP penalty.
The key to this build is that you minimize the effects of your low Wisdom. Do not cast enemy-target spells like attack spells, etc. Instead just focus on buffing yourself, particularly with swift spells and spells like Grave Strike. Then wade into combat. You won't turn undead and you won't be throwing flame strikes, but you will be a rogue with Bull's Strength, Grave Strike, Bless, etc. so that your attacks are decently scary.
Plus, no sense in wasting those unused turning attempts, so pick up one or more of the feats that let you use turning attempts to do other things like directly damage undead, boost spell effects, etc. Sacred Vengeance would probably be your top choice but Sacred Vitality is also good. If you can get a Cha+2 item then Divine Vigor is great. Otherwise Sacred Boost is very nice. (Download the free feat list PDF file from crystalkeep.com to see the full list, and these are all official WotC feats so your DM should approve.)
After that, you're level 12, so look at prestige classes. If your DM will allow you to locate a Worm of Kyuss (from the Age of Worms campaign but you should be able to locate the equivalent in your campaign world from the sound of all the undead there) you can go three levels deep into the WormHunter prestige class, which allows you to sneak attack and crit undead all the time. You can finish up that class, or head on to Hunter of the Dead or Sworn Slayer at that point, and always grab another level of rogue if it suits you.