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This article explains how to use the Holoshield Launcher.
The holoshield is fired from the front of the Holoshield Launcher and it will appear in front of the user. Enemies that try to shoot at the shield have no effect as their shots will be disintegrated by the shield. Also, if an enemy tries to go through the shield, they will receive damage.
To upgrade your Holoshield Launcher very quickly, load the first mission of Kronos, place a Holoshield in front of the turret near the start, then shoot the Shield using the Turret.
Good locations to use this weapon include when going through a narrow path, as the shield sometimes will block almost the whole path. The good thing about the shield is that Ratchet is allowed to shoot through it, and so are his bots. Be aware of that the shield will disappear after some time though.
The Holoshield Launcher can also effectively cover Ratchet's back; however, the player has to be careful as to what movement Ratchet makes as the shield is not very wide.