I am about to take my rightful place among the Nightengales and am going to take out Mercer. i would like to know how challenging will he be if i am a Kahjitt theif who uses Bows and dabbles in the occasional two hander. Should I stock up on the health potions?

Depends upon your class-less level and the difficulty setting.

I had no problem with him at 50-ish on Adept.

LOl my bad I am lvl 21 and on the default difficulty

You're gonna' have to wait for others to chime in because I've only fought him the one time. ...and I don't remember what the default difficulty is - Apprentice?

He was easy to beat, I had a harder time getting to him than fighting him. But that PC was in his level 30's and a dual wielder with Heavy Armor.

He's not that tough honestly, the ruin itself is harder, in my opinion.

I concur with both of these. The location is a pretty tough Dwemer Ruin, as far as these things go. Plus you have the "experience" of having Brynjolf and Karliah with you as an added handicap. If you're careful, and able to stay back when the other two are in melee, you should be okay, but do not let the Falmer get the jump on you.

But Mercer himself? Not that bad at all.

Thanks for hte input guys I got another question though

Did you guys keep the skeleton key for a while and go on a looting spree? or did you take it back to the altar right away?

I was like the guy with the broom behind the horses, cleaning out the dead bodies and dirty old urns.... I let Bryn & Karliah do the work. If you're rich enough to skip the looting, it goes PDQ!
Mercer was quite the man when we went looking for Karliah, too bad he can't be a follower.

Also, I did not keep the key, as I like Karliah and her backstory; wouldn't feel right going against her wishes after the risk she took bringing me in.
(Also, between the lockpicking gear, potions, and gazillion picks at that point, didn't feel that I needed it.)

I kept it for quite a while. In fact, my second character never finished the storyline, so she still has it in the last save. My strategy is to get the skeleton key, keep it until all the misc jobs are done to get to the special jobs, then return it to it's rightful place so that I can finish the questline. Best of all worlds.

I returned it pretty quickly because of this: