[cleveland]

Giant bless rushes are difficult to counter for everyone. For the situation you described, I'd recommend the following:

1) Don't Panic! You've actually got a lot more time than you think: if he's been recruiting 500g Fomorian Kings, he hasn't been recruiting much else, so breaking down your capital walls is basically impossible for the next long while. You're going to lose territory, get over it, it wasn't very valuable anyways on turn 8, so...
2) Appeal to the masses. Diplomacy is your best bet against an early aggressor, specifically "He's attacking me and will therefore take over the world so you'd all better attack him before he gets too powerful blah blah blah!"
3) Execute a fighting withdrawl back to your castles. If you (doubtfully) have the cash & time, build an extra castle in the rear.
4) Garrison those castles with high damage outlay troops. Glaives were mentioned, but don't neglect Wolf Tribe Warriors, Lizard Warriors, and Maul Barbarians, to name a few. You need to do enough damage to outpace that regen. Finance this with 200 taxes everywhere for a delicious scorched earth flavor.
5) Divert research to Construction.
6) Upon hitting Const-4, crap out a bunch of Vision's Foes (huh?)
7) Using your high-damage guys as blockers, break siege with Eagle Eyes/Aim buffed snipers laying down a hail of VF bolts (oh, right, they only have one eye, so we should take it out via irresistible eyeloss, much like Saroumatia's poison bows)
8) Laugh menacingly as those 500g giants go completely blind in 5 rounds even with Arrow Fend, and are then almost instantly felled by your cheap swarms.