I just started to play OW again from the beginning, the first time I played the non-GOG version when it came out. I am currently in the US Mission 13a and I just can't do it. I repelled all early attacks and built an attack force (mainly heavy tracked computer rocket launchers) to erase the Arab base. The thing is that it was the Arabs who eradicated my force with ease: at the southern part of the map right below the ramp leading to their base they have a couple of vehicles (one big gun and two smaller ones) which keep respawning about every 40-50 seconds so I simply cannot reach their base.
I am playing the mission for the second time - the first time I cleared up the base fine, but I lost Forsyth so I restarted the scenario to complete it withput any casualties. I did not have problems with this group then but now it is basically impossible to finish the mission. Any ideas?

Which base do you have problems with?
NW or NE (the final)?

There is only one base in the East - this is the second Alliance mission (13a Peace on Earth) where I am supposed to get Burlak to my base and destroy the Arab base. It is not the base I am having problems with, but the three Russian vehicles which keep reappearing South to the earth ramp defended by three Arab turrets (the entrance of the Arab base in the SE corner of the map). A couple of seconds after my units destroy the three vehicles and I can repair them, a new trio spawns to the exact spot and start shooting at them.
So I can't attack the Arab base since in order to do so my units should go near the Russians - then they will be in a death trap between the Arab towers and the Russian vehicles. I have about 10 rocket launchers but after the third or fourth Russian respawn they are all destroyed.

Oh, I'm sorry, I have confused it with US11 mission.
However did you try to split your units? One group will hold off the Russians and the other will try to attack the Arabian base.

Thanks for the tip, but that doesn't work. The Russians (one heavy gun and two rocket launchers) are so close to the ramp that I can't sneak past them unnoticed, I must fight either the Russians only or the Arabs and the Russians at the same time. If I do the second I will get destroyed, if I do the first, I can easily destroy Ivan but before I advance to the Arab base, they just respawn so I am back at step one.
I don't know how the Russian respawn should be triggered, but since I did not have this problem during my previous attempt, it is either related to something I have done (time limit or the strength of my fighting force) or I just run into a nasty bug. I tried attacking several times using a savegame with similar result, so I guess it is time to restart again :(
Thank you for your help.

Yep, seems like a bug. I remember having no such problems while playing this mission.

Yeah, I just restarted the mission from the beginning and did not encounter the invincible Russians again, finished the mission without any problem. Thanks again for providing advice.
If anyone has the same problem, you can either restart the mission or try this solution (actually I found it the best strategy to complete the mission anyway): build about 6 heavy gun vehicles (preferrably remote ones) and research spaceshifting bazooka ammo. Send your vehicles and one or two bazooka soldier near to the single Arab tower firing from the plateau and destroy it and the nearby trikes. Then send your vehicles as close to the ridge as possible and fire at them with space shifting missiles. Eventually one or two will teleport to the plateau, keep firing at the remaining ones and transport them all. If you have all of them at the plateau, kill the nearby gun, the mastodon and go south to finish off the Arab base. This way you will not encounter the buggy Russian vehicles and avoid the tough Arab towers and trikes at the ramp.