Understandably, some players may want to do it fairly but I found a cheat to use against those who are struggling and want to skip it.
1. Equip Unrelenting force and make sure you know all three words of the shout.
2. Lure Mercer up to the top.
3. 'FUS RO DAH!' him off, it kills him instantly.
It worked for me and I hope it works for you guys :)