The Death Knights are truly the thing that makes this battle so difficult.
Good timing with the staff is crucial here, but even then, there's the probability that they'll block your attack. It gets impossible if he's summoning death knights like you say. Most playthroughs where I see this boss beaten usually always seem to co-exist with fairer Death Knight spawns.
Also, be sure not to exaggerate. Are the Death Knights really spawning as much as you say? Or do they just "feel" like they do. It's easy to get caught up in frustration with this boss and blow things out of proportion. Anybody who saw my streams are well aware of how I blew things out of proportion. Just a little. 