Here are just some of my thoughts:
I think you should replace the guardians with 5 dire avengers each simply because it's cheaper.
Also replace the small jetbike squad with 5 rangers, because with two avenger/guardians in serpents you have enough mobile objective grabbing/stealing. The rangers can sit on your back field objective and are difficult to shift once they go to ground.
Another thing: what is your plan with your farseer? I assume you want to guide your walkers because they have 24 shots or what? .. if so then why the large jetbike squad? .. if they are just to keep the seer alive then rather take the mantle of the laughing god and 5 more rangers.