Two recent patches, which closed the possibility to attack low levels and nerfed income from last campaign mission to only 1,000,000 (from 10), made this achievement really time consuming. Instead of suicide bombing oil rigs and factories in high level bases or waiting 1 hour until your troops rebuild while you can be attacked in the meantime, there is simple solution.
Firstly, upgrade command bunker to level 7. It costs 1,500,000, but it isn't that hard to collect it. Finish campaign if you need resources. Then make sure that every supply building is at its max level. You need 3 rank five oil rigs, 1 rank 3 airdrop and 5 rank seven factories. Double check if your supply depots have enough space for 4,000,000 resources. Don't spend gold, you need exactly 700. You can build 1 supply depot at rank 9 and 2 at rank 8, which have 7,002,000 capacity. When you are ready, buy four week camo from camouflage section at store.
During each hour your airdrop produces 3000 resources, 3 oil rigs produce 6600 and 5 factories 6500. This means that during 28 days you can multiply those values by 24 and then by 28. Oil rigs would produce 4,435,200, factories 4,368,000 and air drop 2,016,000. In total that's 10,819,200 resources during 28 days. This means that you should have enough resources to build both command bunker rank 8 and rank 9 (10,500,000 in total). All you need to do is log in for a minute every 9,5 hours (air drop fills itself up in 10 hours, so this is the important time rate when you have to collect resources). Preferably log in at the morning, then after going back from work/school/anywhere and finally when you are about to go to sleep. All takes no more than 5-10 minutes each day. You would get additional supplies from random drops and supply chain, but those are bonus. If you don't want rank 9, you can also spend it on anything.
Just one huge reminder, DO NOT think about attacking anyone during that 28 days period. No matter how much resources your enemy has, you cannot attack him or your camo is lost. Just stay away from attack button and you'll be fine.