right i did it by taking the worm over to the 7.
collect the health crate, go between the 1 and the M and then blowtorch through the 1 in the first go and hop up to the corner of the 7.
now dont use your bunker buster , airstrike or homing missles until you think you can get a definate kill.
just keep poppin out through the tunnel you make, lobbing a grenade or whatever and then run back in and up to the corner. dont forget to use the different grenade timings (Y) and also when you get closer to the end use the girders and put them as close to the enemy worm(s) as you can to block them into the landscape. you can just box them in, giving you time, if you need it to jetpack or rop over and use the high explosives
you should get it easily now