So I reached the volcano levels, and entered this big open battlefield with a lot of enemies in it. There are little monsters jumping around and the heavy dudes with rocket launchers. I get killed over and over, and I don't really know what to do.

I try to stay near the gate, and wait for the heavy rocket monsters to come to me. Then I shoot them in the head with a machine gun. It doesn't always work, as I have no chance of dodging the rockets.

If I try another strategy and simply run to the other gate at the opposite side of the field, I get killed by all sorts of gunfire.

It's damn near impossible. Any suggestions?

a good amount of tactics talked about here: http://www.gog.com/en/forum/far_cry_series/the_last_level_wtf

Do the following. Take the M249 Saw Mg, sniper rifle with you. If you enter the battlefield immeadiatly go down and sniper down one jumper directly in front of you and you can also snipe down the other to the left. Then crawl into the edge just where you entered the area and go in the knees and also lean with the M249. Now just wait for one rocket monster to come after another. and just shoot at the head. They will go down after around 40-50 rounds. After you killed some of them. try to nipe down the rest of the jumpers. That is the basic strategy.

Here is a video guide for the solution i do it quite similiar:

Search on youtube for: chakram far cry vulkan teil 2.