Mangler

Ranged weapon is the easiest way to beat it.

Its hits have pushback which makes it hard to fight him from close range (you get pushbacked even with a shield). If you have a priest in your group, casting divine armour will completely negate this and make them a pushover.

It's still possible to kill it otherwise by hitting once or twice, moving on his other side when the hit animation begins, rinse and repeat : the timing isn't trivial to get and will change according to the weapon used (with 1H sword, i could give 2 hits per move), but it's not too hard either. Be careful : getting hit once with this method can easily get messy: it's not easy to use the few time you have after getting hit to avoid next hit. You can check the following video as an example.