You will need the following: L3 of multi strike, berserk, and slow time. when you begun the battle, cast slow time, then berserk, then multi-strike, hit him with the sword of aeons or avo's tear.
when he lands again, repeat this process and you will defeat him with greatest ease, i beat him in under 2 minutes with that technique.
p.s. do not worry about the minions and summoners, just focus on the dragon
happy slashing!