The Shield Generator is almost useless if you use an attack strategy that fights its way through the base. If the defender chose to have the shield instead of a Prototype Defense that shoots by itself or increases damage, the player will fight through the base, the shield generator will be taken out easily and the shield will not hinder the attack at all.
Timing is very important when doing this. Be sure to know how many times the Warriors will have to hit the generator to destroy it, so that you are ready to cover them again immediately after the generator is destroyed.
Shield Generators defended by only a few defenses can be destroyed by using Artillery and Barrage on the defenses guarding the Shield Generators, then using Critters to pick off the Shield Generators.
The placement of the Generator is important; an undefended Shield Generator can be easily picked off by any troop, wasting the shield around the Headquarters. Protect it as much as possible to prevent enemy players from easily disabling the shield.
Placing the generator behind the Headquarters can make it harder for troops to disable the shield before attacking the Headquarters.
If the attacker decides not to attack the Shield Generator and instead decides to wear away at the shielded Headquarters, the Shield Generator will grant your defenses lots of additional time which can be critical to a successful defense.
If you decide to have more than one Shield Generator at a time, spread them out as much as possible to prevent attackers from easily destroying them all.
At Mark I, the Shield Generator is a light blue machine with a black top that has a glass dome containing an electric coil at one end.
At Mark II, the top part of the device has a second glass dome containing another electric coil at the opposite end.
At Mark III, the Shield Generator is made up of what seems to be two Mark II Shield Generators placed right next to each other.
The mark III Shield Generator appears to be two mark II Shield Generators placed right next to each other. However, the mark III Shield Generator improves the Headquarters' health by 150% whereas two mark II's would improve it by 200%.
The Shield Generator and the Ice Statue's health improvements are multiplicative; the shield's strength is its percentage value of the Ice-boosted HQ. This is unlike the Damage Amplifier, whose damage improvements and the Ice Statue's are additive.
Multiple Shield Generators have an additive effect to the shield, i.e. two mark IIIs would make a shield at 300% of the Ice-boosted health of the HQ.