The P-Shield is the strongest mobile shield generator and is considerably more cost effective than the Cybran Adaptor for the shielding it grants. The Adaptor has many other uses however.
Each individual shield is no stronger than a unit's personal shield, so be sure to liberally build them to boost the toughness of your units.
The P-Shield has the same speed as a Titan Assault Bot. One strategy is to group the two together and harass the enemy's base since almost no land units can catch up.
By default the shields will sit at the rear formations, which in many cases will mean the front line units are out of range of its protection. It may be wise to keep them in a separate group in the middle of your attack units.
The P-Shield is an excellent choice for factoring into the Noah Unit Cannon's unit build orders. As it takes many seconds for the 21 units to enter into the battlefield, these small health-boosters will keep the first few attack units alive long enough for the rest to bolster them if sent directly into an enemy base.
The 2 standard approaches for breaking shields - missiles and closing range - apply to the P-Shield. However, since most UEF units have a fairly short range to begin with, your best bet to counter a P-Shield is to simply close distance and get inside the shield. Only use missiles to overwhelm a P-Shield if there's a small, stationary group of them guarding something. Otherwise, closing distance is the superior tactic.
Whilst the shield is obviously able to defend multiple targets at once in a way that a personal shield does not. Be wary that it also projects a large target and you may find your shields collapsing due to fire that wouldn't have hit your units anyway.
Whilst the shield is substantially weaker than those produced by Shield Generators, the P-Shield has a rebuild time a mere quarter of that of its stationary counterpart. In protracted battles, this can mean much more prevented damage than its shield strength would suggest. This also can make P-Shields instrumental in protecting your main shields from artillery fire if they are down and recharging, as the P-Shields will recover faster and shield your other shields from the artillery.
If you are turtling as a UEF player, build a few P-shields and set them to patrol around the inside of your shell (the base you are turtling at).