These missions are very hard, almost impossible to complete so I'll give you a few tips:
1.The missions were designed to be played with health regeneration so use DYOM V6
2. SWAT TANKS and SEA SPARROWS are ALWAYS controlled be terrorists and are equipped with long range mini-guns and rockets.
3. Only explosives can damage tanks, so try to collect as many as you can.
4. During defensive missions find a good sniping position and try to stay under the enemy helicopters to prevent them from firing their rockets at you.
5. Enemies with sniper rifles and mini-guns can be anywhere so move very carefully.
6. Use rockets to clear ambush points.
If these tips aren't enough I made this walkthrough:
http://www.youtube.com/watch?v=tRUj7OTGE-w