You absolutely need capped res' for starters.

Then, the super bosses ignore most normal stats etc. so you need backup plans and things. Ravager is super hard for a lot, as he tends to favor 'face-tank' builds most, in terms of beating him.

He's got lots of life reduction % attacks and so on, I fight him with a Warlord that's got ~70-80% life reduction resistance and so on. You want alternate means of healing aside life leech too, if that's what you normally rely on, and just tons and tons of mitigation. Again, Warlord, shield tank etc. is how I do him. I've seen an AAR build do it though, but it was running for it's life the whole time ha.

You can look him up on Grim tools also, to see all the stats and such to his attacks to get a better idea what he'll throw at you, but he really, really favours face-tank, tank-supreme style builds to take him down.