Orochi.
We have a winner!
Clues explained:
#1 I count Hydra and Jörmungandr as my cousins.
- (Yamata no) Orochi was a great 8-headed, 8 tailed serpent of such immense proportions it could lie across 8 mountains and 8 valleys.
#2 Some might call me a Great Wyrm, or even a Dragon.
- A big snake like that could certainly be called a big dragon. Furthermore it's sister was the Sun goddess, so the moniker 'great' certainly is apt.
#3 The battlemech that bears my name is ironically armed with my nemesis's weapon.
- The Orochi's signature armament is Thunderbolts. And Yamata no Orochi's nemesis was the Storm God. (yet another parallel with the midgard serpent..)
#4 You'd be hard pressed to describe me without using the number 8.
- See explanation for #1 :)
#5 If I resemble one of the Un/Reseen, that's because I was designed to replace it.
- The mech was designed as an updated Longbow.
#6 If you're tired of longbows and arrows, I might be what you're looking for.
- A sneaky reference to #5. Also to help eliminate O-Bakemono as the answer.
#7 I hate Anti-Missile Systems.
- Wouldn't you, if you were armed with twin Thunderbolt-20s? Still it's not as bad as it was pre TW.. at least AMS has a 50% chance of not shooting a Thunderbolt missile down now :)
All you False Son.