Skyrim: Killing Mercer Frey, how?

Ok, i had a bit of a problem with this. Killing him for me was fairly simple. I am mostly magic so i would just run around and shoot a dual charged firebolt at him when he wasnt close enough to hit me. AVOIDING HIS DAMAGE IS A MUST. Because it seems when he hits you he gains health.

Now, for the hard part. When you finally slay the bastard the entire place will fill up with water and you have to find your way out and of course the original door doesnt work. I found it by chance and maybe im just dumb because it took me so long, but there ended being a hole in the ceiling of the place which to me was not noticable at all.

Happy killing!

Skyrim Mercer Frey

1. Yes, you do get to kill Mercer. And it's a good idea to take the two Eyes of the Falmer off his body when he's dead. 2. Well, technically yes, but a number of things have to be done in order to get to that point. One, you have to choose a sign of Nocturnal to operate under. This is part of the Theives' Guild questline. Two, you have to complete at the very least, twenty small jobs for Vex and Delvin, but it's not as simple as picking just 20 jobs. I'll give you a brief spew about the small jobs. The Small Jobs consist of several small petty thievery jobs that Delvin and Vex offer that can be completed for payment and standing in the Guild. They will be done in on of five cities: Windhelm, Whiterun, Markarth, Solitude, or Riften. As the Guild already has standing in Riften, you do not want to get any jobs taking place there, but grit your teeth and do them anyway. You get paid to do so, so you might as well make some money for five minutes' effort. Personally, I find the Numbers jobs to be easiest, but there's also the Planting job (place a stolen object on someone via pickpocketing), which is easiest for those with low Sneak levels, as getting caught by guards does not trigger failure like some other jobs do. For completing five small jobs in each of the other four holds (e.g. 5+ in Whiterun, 5+ in Windhelm, 5+ in Solitude, and 5+ in Markarth), you unlock a special quest that adds another fence to the guild, as well as merchants within them. There's also a special quest for Tonilia that you can do to unlock the Kha'jit merchants as fences as well. By completing all five of these special quests (which includes all their prerequisites), you start the quest "Under New Management," which instates you as Thieves Guild Guildmaster, which gives you access to the Tribute chest and the Guildmaster's Armor, which is a better version of the Thieves' Guild Armor.

Thanks for the answers!