Heh, now the question will be what is everyone's strategy for knocking down a mean defense like this.
Thats going to vary a lot depending on what general you play as, but as a general statement massed artillery mixed with infantry works well. The HellfireII base has long range and high damage, but a piss poor reload speed and can hardly target infantry at all. If you scatter infantry around your artillery pieces they will draw a lot of the missiles and will protect your artillery. Or, if thats too much trouble, just swarm it with numbers. It can't destroy them all at once.
The picture perfect SupW base defense line would use all 4 defense types... Patriots and 30mm guns out front with HellFireII and SAM sites right behind them. now, 4 defenses at 3 power each will require a whole power plant to themselves, so the enemy can't have too many of these defense lines. Even if he has 10 power plants to feed 10 such groups for the same price you could easily field 30-40 artillery pieces to shell him into submission. If he builds all Hellfires that wouldn't work too well, but then it's infantry or air strike time.
Also, don't forget we're talking about "how to battle a turtle" here... if he's using his hard earned doh to build a wall of defenses this means you can build units and claim the resource points all over the map and have a much higher income to spend on teching yourself up as fast as possible.. T2 Tanks, Planes, and artillery units make short work of T0-T1 defenses.
Also the defense wall is power dependent, and as it techs up it becomes even more power dependent. Every general has some kind of attack that can hit those power plants in advance of a major attack. Turtling works well vs the AI but not in PvP.
A short break down by General focused on T2 (T1 is fairly universal and fits the above description)
AirF: T2 has a Laser Point defense upgrades, cruise missile bombers, and Auroras.
SupW: Get your supers online first... Use Auroras to crush his power and super weapons
Laser: Mithrals can protect you from both the hellfires and the Long Arm defenses while you're artillery units destroy the defenses.
Robot: A bulk swarm of spider mines can work wonders. Laser point defense drones in a swarm can protect you from missiles while you use artillery
Nuke: Nuke cannons have some of the best range in the game, easily owns up up Hellfires.
Infa: 100 mortar troops fear no defense line. T2 artillery spotters out range the defenses.
Flame: Pheonix bombers and the Smerch can clean house on them. Flame does high damage to buildings.
Tank: This is probably the toughest fight. There is a natural degree of Rock paper scissors in ZeroHour, and from Vanilla 1.0 SupW has been the bane of Tank. At T2 you can use mobile supers to target power plants. Then roll tanks in.
Stealth: Mass Sabotage, or Saboteur can shut down power before a frontal attack. Or, you can let them bunker down and leave all those base defenses up and running for nothing, never let him shoot anything while you spam powers from the black markets and let your monuments slowly ravage him. Your Jammer station will keep you protected from super weapons and powers preventing any meaningful retaliation.
Demo: Swarms of suicide units are cheap and effective, high end artillery is effective. You can also use Mass Sabotage or the Saboteur spawn to shut down his power just in time for a huge suicide wave that will clear out the defenses.
Chem: the Toxin tree is very effective at shutting down buildings. If you can get even a few T2 or T3 artillery shells to land in the area you can start shutting down the defenses and have your way with his base.
Aslt: No one has more artillery and counter artillery than assault. Build your Faithful cannons and shell him into submission.