Please offer some advice.

Take two different armies and go for it. On Champion difficulty, it's quite easy. The walkthrough is described on the AG.ru website (though I don't remember which level it is). But there's one thing to keep in mind. The 5-day countdown starts from a specific point (I think it's when one of the heroes opens the passage, but I'm not sure). After that, you can reach both enemies (one on the right, the other on the left), but you need to hurry – if you linger somewhere, you might not make it in time.