I don't imagine Ed Miliband reads my humble election-analysis blog, but just in case: here's some advice ahead of a debate that holds a number of potential pitfalls for him. There are opportunities in this debate if he handles it correctly. After all, his personal ratings have improved considerably after each TV appearance so far during the campaign and the same might happen again, if he plays it right. But here is some free advice for you, Ed!
1) Prioritise the "enemies". The media are going to try and bill this as Miliband vs. Sturgeon but ignore this. The chief enemy is the only other possible PM: the absent candidate! The absence of Cameron and Clegg on the panel is huge open goal. Take every opportunity to attack the government's record: nobody's going to defend it. Don't overplay it, but make sure you remind the audience that Cameron and Clegg could have been there but chose not to be. After Cameron, the next "enemy" is Farage. I know the SNP are a bigger electoral threat to Labour, but Farage is the ideological enemy, and Labour voters, members and supporters will want to see you challenge him vigorously. The Scottish leaders' debate will be the "opportunity" to tackle the SNP - keep your focus on the national picture in this debate.
2) Lead the progressive bloc. You've done well showing the public that you can be tougher than they thought, but don't imagine that people want to see you beating up on the minor parties. It's one thing standing up to Paxman and laying into Cameron, it is quite another to attack Bennet, Wood and Sturgeon. There was some positive public response to the fact that these three cooperated on areas where they agreed. Why not try a bit of the same? A bit of "I agree with Nic(ola)" could be worth a thousand attack lines that strike the wrong note. If you lead the other three in the charge against both Farage and the absent coalition, you should be able to harvest "I agree with Ed" lines too.
3) Don't EVER attack any of the others from the RIGHT. Just don't do it! We've seen your notes from the last debate, and I'm worried you're getting some questionable advice in this area. If you attack Sturgeon on the issue of Trident, you must remember that most of your PPCs and many of your voters agree with her, not you, on this issue (including me, as it happens!) It's too late in the day to change your policy on this issue before the election (sadly) but it would be hugely counter-productive to use this as a line in the debate. It would alienate potential Labour voters who are wavering about voting Green or SNP (or elsewhere) and it won't win a single vote from a Tory! Similarly, don't get into a situation where you're defending spending cuts against anti-austerity arguments. If you can find an angle where you can show the SNP to have been pro-austerity / pro-"1%" then use it, but otherwise focus on points of agreement.
4) Stay positive. In the interview with Paxman you put forward some positive, progressive arguments relating to immigration and these are the ones to go with in the challengers' debate - don't try and sound "tough". Make sure you make the point about the positive consequences of immigration and your own family's migrant story. Make sure you make the point about ending zero hours contracts and cracking down on exploitative employers, rather than attacking migrant workers. This is a strong line of argument and is a good attack line against Farage. When he says he wants to stop migrant workers undercutting "native" workers, ask whether he supports you on zero hours and the living wage. If not, he has zero credibility on the issue of low pay.
5) Don't over-react if you're not "the winner". There is a reasonable chance that either Farage or Sturgeon will come out as "the winner" of this debate in the snap polls. These polls are more-or-less irrelevant so don't panic about them. You and the three women are essentially fishing in the same pond whereas in this debate Farage will have a pond all to himself. If he doesn't "win" then we can conclude that the wheels have come off the UKIP bus and I wouldn't be at all surprised if he doesn't even get his Thanet seat. You might well be seen as the "Westminster" "establishment" candidate and therefore left-leaning people who are polled might well opt for one of the nationalists. This doesn't really matter - most of them can't vote for them and they know that the choice is between a Cameron government and a Miliband government (as you should remind them during the debate).
So - there you have it! Take this advice and you can get something out of this debate. Ignore it, and you'd have been better off joining Dave and Nick and watching it from the sofa!