This will be a more harder one. You will need to protect Reti. He will go on a set path, leaving far from the path. The first part is easy at least.

First rule. Kill everything that shoots.

These are the only ones you need to focus on killing. You may take your time, since Reti is on a set path. Play as safe as you like. When you reach to the o2nd canyon, kill everything there and wait for Reti.

2nd rule, wait for Reti. Becuase he will die if you go without him at the very end. Since he takes instant damage. If you go beyond the 2nd canyon.

Here comes the hard part. Your real time begins here. When Reti is around this view, It is 100% safe to rush the next canyon. However it is very narrow. Do it any way you like, BUT I recommend you try staying above. slow a bit down at these parts. Or risk it.... After that, you will meet up with the group. Kill everything there, and Reti will teleport. Skiping some time. You kinda need to rush this part before Reti makes it to the end. Proceed to the next Canyon.

Do the same thing again of clearing everything out in the path. Try to take less time, since Reti will try to land as soon as he makes it trough.

When you make it to the last canyon. kill the tanks first before they do enough damage to the base, and end the mission. Battle droids will spawn at the far end of the canyon. some will walk along the walls too.

Good luck.