I was able to down Gilgamesh on my 2nd try, all missions. I would have had it on the first but I forgot to seal lightning after the 50% threshold and could not recover. I followed the same tactic and team as Evil Laughter01 on youtube. Actually, my team was a little less geared since I did not craft as many of the new rings. I ran it with Cecil (6.5k hp, 460 def, Golem), Refia (DC, Ramuh), Rain (DW Survival Edge/Excal, Diabolos), Bartz (Chirijaedon, Odin), and friend Noctis (Excal, 880 ATK). DC on Cecil would have made it easier, but it was a pretty well controlled fight. I lost Rain a couple of times during the final phase (stupid cover ability), but luckily was able to seal the right elements between Bartz and Noctis' lightning flask. Bartz was the MVP of this fight with his dispel (Fingersnap) and Life Giver. Obviously, could not do it without Noctis leading the damage. Cecil basically just did provoke and guard the whole fight and he was targeted for KO at 2/3 transitions.