I have to clean up what is left of the enemy ...many underground turrets, but they are very strong against my forces. Anyone know what to use or what tactic to use against turrets. Cant use combots underground...

Is it a campaign mission? Then your arsenal might be limited.

There is a Tectonic Torpedo Turret attachment for Rail-Gun platform. It's sole purpose is to attack underground with very potent Tectonic Torpedoes. They can even be launched from the orbit layer of the battlefield. Be advised though, upon detonation, the resulting explosion will damage units and structures both on underground and surface layers. Also, it should be available to all CorpoNations.

Another little friendly advice, please refer to the manual, provided with the game - there are interesting things there that might help you with situations like this one.

Hope this helped.

Artillery tanks and your own turrets are probably the 'quickest' way. Building Recon Poles above the enemy will help significantly. Large swarms of ordinary tanks and (in the later missions) the Rail Gun's "Tectonic Torpedo" mode also work, though they're both going to take a while to do it.

In general, the main suggestion I have is to be slightly more aggressive in your expansion. You don't want to set the AI off too early, but if you wait around and leave the AI unhindered too long you'll end up having to do this in way too many missions. Worst case scenario you do poke the AI too early and need to reload a save or restart, which is annoying, but in the larger maps it can take hours to dislodge the AI from the underground so you'll still save time.

edit: ha, should've refreshed the page. I'll add that you should build multiple Rail Guns, since the reload time is quite long and turrets can take 3+ shots if they're in defence range and not hit directly.

Also, you may try to spam some Nemesis suicide trucks through the Elevators as a last resort, they can clear out quite a lot of enemies if used correctly.

Thanks for the tips, it looks like I'll use artillery tanks. This is the 3rd mission.
I did read the manual BTW.

No prob, that was just a friendly reminder. Best of luck clearing them out.
What CorpoNation's 3rd mission, by the way? RimTech?

It is a fair-point. Yeah, Rimtech. So the beginner missions I suppose. Never played this game before but I am a retro strategy gamer. I have every RTS on GOG.

Another thing you can try (though only if you already cleared out the tanks and there aren't too many turrets): get 1 hover veic and send it in first; when the turret targets it, manually dodge the slow moving shots and order your tanks to destroy the turret.

(I mostly use this to destroy enemy turrets in orbit, using 2 hover veics; it might work underground too though)

zerg rush tactics