Why is there this assumption the Greens will attack Labour? I'm sure they will "attack" but I don't see any evidence to suggest they will single Labour out.
The bottom line for me is Cameron is destroying the NHS, presiding over a failed economy and is totally incoherent with his immigration policy. These are the issues and knows he will have Labour, Lib, Greens and UKIP all gunning for him and is running scared.
I don't think they'll attack Labour any more than the other parties, but it's definitely Labour votes they are after. The best tactic to get them though might be to attack the Tories even more aggressively than Labour are doing, saying they offer the only genuine, radical alternative (which I guess kind of is an attack on Labour).
They might win some votes from the Lib Dems, but I suspect that anyone who's going to walk away from the Lib Dems has already done so. I don't see anyone jumping from Tory to Green, and they're so polar opposite of UKIP that that would be a very odd switch (bar those that just want somewhere to register a protest vote) so their best bet is to take left wing votes from Labour.