This level is really beutiful and interesting and not defensive. Attack and counterattack but have a fixed line of heavy units like beetles! The only point is that your resources are limited and you shouldn't attack without overwhelming forces.
Two camps, yours and symmetrics, are connected by the sometimes extremely narrow and sometimes relatively wide passage. This passage has 3 oil patches which you would have to use once the two patches in your valley run out. Also, you cannot upgrade to crazy harries or missile crabs. So, the first thing is to defend the entrance to your valley with beetles. This will consume one oil patch and you have to switch to another and need one more tanker (it is better to build a power plant closer to that second patch).
Then, build at least six beetles, oil derrick, 5 mechanics and accumulate at least 2500 ru. The enemy will constantly attack with infantry and barrage crafts. Follow them after one of these attacks to the next oil patch inside the narrow passage. Build an oil rig and two towers and place mechanics near them since the enemy will counterattack very soon. 6-7 beetles and two grapeshot towers are enough to deal with it almost without any damage.
Use up the second oil patch and build again 6-7 beetles, oil derrick, mechanics and 2500 cash. After one of the attacks go through the passage to an opening with the oil patch and build oil rig and two towers. Move all your beetles to defend (at least 12-15 because here the passage is more wide). After the first attack move all your forces to the next oil patch by the entrance of the narrowest part of the passage. It is very convenient to defend here since the enemy units will appear one-by-one from the passage and are easily destroyed by these beetles.