I'm a bit unsure about the stability of this list...
It looks good on paper, but with dice rolls, the Tyranids could smash through your Tacticals quickly, especially the termagant's and other when they use the rule whereby they can redeploy after death.
I guess it would depend on objectives, in a CoD battle, this list would take the nid's easily, though you'd have to be careful with stratgems. In a grudge match, you'd lose badly. If using VC's, you might win, depending on luck. I honestly can't see ways of improving it though, as I've never really put much thought into the whole Marine horde concept. All I can say is playtest, playtest, playtest.
I must say though, use tactical maneuvares to your advantage. Force the nid's into narrow corridors, like streets or ravines, and pour bolter fire into them.
Oh yeah, and use flamers, loootssss of flamers.
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