one of the easiest ways to do this is comlete all the quest you can first lol, it helps, but as for a stratagie

make 4 groups
2 mele (4 units in each, all tanks and attack, use the vangaurd formation)
one magic(4 units, have som good attacks, number 5s help.
1 group of fast people, and you use them for healing, us the line formation, it increasse mobility.

make sure each union can revive.

then when you start bring out cyclops and kill the hypnos, then have cyclops engae hellsgate and keep him busy with your 2 tank groups, have your healers standby unless they have long range attackes, and hit them with you best mystics with the mystic group, you can get a round od deadly hits in, then pull them out to heal, repeat untill process is complete.

hells gate has about....100k in health(thats where i added my figth around, my tanks dealt aroun 10-15k here and there, but theres a good chance he has 200k health, not sure)