Can someone share some tips on Stage 18 (Lightning Mountain).

I have to destroy 4 fighters, but I'm only given 3 copters. So what happens is, 3 of the fighters come and destroy my copters (I then immediately shoot them down with my two anti-air missiles), but the 4th and final fighter just flies away to the southeast corner of the map. And then it just sits there cowering and never comes to my units. I don't have an airport and I can't cross the river so I don't know what to do. >.<