1 balanced 55 group could easily take Vilefang down. Getting to him is harder than killing him. Personally I have more problems with retards tripping the dragon construct spawns than I have ever had in killing vilefang.
Not quite, just grab a 55+ group and have them invis till you get to the front of the arena. When the puller pulls the first pair of giants, root the others. Take out the first giant, then the second etc. Rinse and repeat till you get to Gkean.
...or find a giant allied friend. Sow him up before zoning in, run him to Gkean, shoot him with an arrow and have him run to any zone. Have your team take Gkean out.