You're a beast! It's only a matter of time...
You definitely need the right balance of damage dealers and tank/healing. You need to survive long enough to kill the gryphon, but deal damage fast enough to get the gryphon before it starts to use the Gale skill. Your best bet is to max out your classes, get the best late stage equipment you can and try to promote your units as early as possible (a Fighter that levelled up from level 1 is significantly stronger than a Fighter that was promoted from a Squire at level 12).