When it comes to these 'win or bust' matches, all you can do is take the options, make your decisions and let the players get on with it. Apart from formation and orders there's nothing more you can do until the match has ended.
Take a peek at the table in my signature - each of those seasons contained one or more of these 'what do I do?' situations - sometimes it led to a red box, others to a white one, but in each of them I gave it the best shot I could do in the situation that presented itself. You either make the right decision and succeed, the wrong one and fail, or you had the right idea but the game didn't go your way. Enjoy success, learn from mistakes, and if it goes wrong, there's always next time...
Yeah, it sucks to be in this situation - and it sucks if it doesn't come off - but we've all been there and will have a sympathetic ear if the worst does come. Good luck.
Any formation or tactics advice given is based purely on experience with my teams...