What is needed to defeat the base: (The base being the one where the outer most defenses are mortar and Hellfire turrets with the 4 bunkers in a symmetrical fashion, kinda like a diamond shape)
Level 9 Elite Hellfires
Level 8 or 9 Elite Warhawks
Gatling Trucks (The higher level the better) (4 is the preferred number)
1) Use EWH to destroy the vulnerable towers that will threaten the EHF when they go to attack.
2) Use Gats as a shield to take the sniper fire from the South East Bunker while EHF''s take out the bunker.
3) Next, use EHF's to target their towers while remaining out of range of the Plasma and Barrage towers. The splash damage should destroy all nearby flak and gat units around the barrage and plasma turrets. (If you fail to destroy all AA units, you will have to find a way to destroy them, use whatever you think is suitable to take them out without loses)
4) Use EWH to destroy the turrets that are now exposed to air attacks.
5) Nearly there now, Now this part is the tricky part. Spawn ONLY 1 EWH and send it around to the OPPOSITE side of the base, making sure that all the flak and gats are *aiming* at it. (Since they are on stand ground they wont move to attack it, but will remain Locked on unless another air unit gets too close.) Now, spawn the other EWH and go through the gap in their defenses that has been created and begin attacking the CC.
If done correctly, the flaks will not turn around to attack and in about 2 minutes the CC will fall. If done correctly, the only losses you would had suffered would had been from your gates which can be micro healed in less 10 minutes.
Notes: There are many other ways and variation to my own strategy to defeat the bases, but ive found this suites me well as it offers a simple strategy to quickly and swiftly defeat the base.
I hope this helps and good hunting commanders

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EWHs should be used to take out all non-AA turrets. To take down these bases you need to eliminate the bunker first. The EWHs, they will hurt one part of the base where there are mortars, rbs, hellfire missile turrets, plasmas. Again, you have it right to try out one EWH to see if you do or don't get smashed.
Everybody has to be aware of the flaks. They are a pain in the ****. I won't elaborate further but i will say you should consider using vehicles with much stronger armour than gats. Personally i use megas, but that's just me.
Event Rewards to get: the FAV, FAV schematic, elite gladiator, elite suicide truck, gat schematic, spectre, viper and warhead.
Flaks are a threat, but if you are able to get them locked onto 1 of your EWH, then the CC won't be a problem, unless they get too close or they damage the flaks whioch will result in them tearing up your EWH's and giving you a hefty repair time.
One thing about your theory is that the speed of the gats gives time for the speedier, multi and more powerful sniping fire to trash the gats and that would mean you need about at least 3 or 4 before the snipers start hitting the EHFs.
If it is the base layout i think you are talking about all of the range turrets that exceed the EHFs range. With that layout you can safely take out two bunkers, say the west and south. There are mines at the entrance to one i take then you get rid of all of the turrets on one side (that would be the side you attack the CC) and the other where you get rid of the ones that can hit your EHFs while they are decimating the CC.
Personally i use Megas to withstand the sniper fire and while they aren't at lvl 8 or 9 they are able to take the damage enough for the EHFs to wipe out the bunkers. My EWHs take out those turrets that could inhibit the progress of the EHFs. Oh and the flak guns haven't given me too much grief as a result of this strategy.
Event Rewards to get: the FAV, FAV schematic, elite gladiator, elite suicide truck, gat schematic, spectre, viper and warhead.
If you use 4 gats, your hellfires won't be injured. I personally use 4 gats as the snipers usually destroy 3 and injurer the 4th which is why having 4 is a good bet to keep your hellfires safe.
I don't use megas because of the repair time. 4 gats being repaired is far quicker then a single mega. A destroyed gat has a repair time of about 7-8 minutes which means that only 2 minutes of repairs are needed before you can micro heal the last 5 for free. So 2 x 4 = 8 minutes of repairs. A mega will take anywhere between 15 - 30 minutes of repair depending on the amount of sniper fire it takes. So its easier to use gats or other quick repair units as shields.
Event Rewards to get: the FAV, FAV schematic, elite gladiator, elite suicide truck, gat schematic, spectre, viper and warhead.
Lvl 28
Total Anarchy
Sec 157
UNITS UNLOKED
Attack Dog, Gattling Truck, Lazer Tank Warhawk, Hellfire
Don't you read the previous comments, don't keep telling us about the lack of level 40 bases, the devs know about the problem so quit bugging the rest of us with these useless comments.
Same goes for you Craig Mayham.
You can use a Mega tank but it has a few disadvantages:
1)Repair time: anywhere between 20-30 minutes.
2)Size: The size of the tank means that it is possible for it to be hit by the RBT or PT if moved too close because when moved, even slightly, it moves a lot forward, not just a little bit.
That's why using Gats is the preferred option, because of the quick repair times, small profile and quick mobility.
1 tank isn't enough. You need back up. Also you need air support. Now, 4 gats are needed if you want them to and those 4 will get damaged. 2 Megas is enough. Yes it's a lot of damage, but it's worth the effort as long as you get things right.
2 bunkers on one of the layouts are the only ones that are needed to be taken out and they have to be linked like west/south. But as MetalStormV said, forget about attacking these bases because you need HFs or EHFs, preferably EHFs for their splash damage.
Event Rewards to get: the FAV, FAV schematic, elite gladiator, elite suicide truck, gat schematic, spectre, viper and warhead.