PWR Loaders have three "claws" on each hand, which they spin around as they slowly walk towards players. Any bullets fired will be deflected, even hitting the player that fired them. At close range, they will try to melee the player, which is the perfect chance to strike, as their hands won't be spinning.
It is important to go for their feet at this point when trying to take them down. Be careful to not let them get too close because the arms will be able to deal serious damage if they are still spinning when they get to you.