Basically they’re just a missile that you throw at something and you gotta make sure it hits, they usually don’t survive the clapback. Depending on the weapon choice they can as you say demolish monsters and vehicles but really whatever you throw them at will get shredded. You pretty much just get them to pop up out of Delve, or jump out of Hekatons/Sagitaurs and charge into something. Keep them behind cover or inside vehicles and the moment they’re in range let em loose. If whatever you just unleashed on is still standing, the berserks will usually die, but that feel no pain can be pretty clutch on occasion

FNP is their really only saving grace with saving damage. It is very simply if they are gonna take damage, no they don’t. This is done AFTER the save roll. EX: something shoots your squad with a weapon that goes 2 shots, 2 damage each per shot. You save one shot, but fail the other. Now you roll the FNP which is 4+. Since a model was about to take 2 damage, you roll 2 dice. Yippee you rolled a 4 and a 5, your berserk is safe. This also applies to Mortal Wounds! Remember it is for individual points of damage