Singularities and Krieg mix very well. Got to lvl 38. I maxed out his middle skill tree to turn into a BA Psycho and if you do it right you never run out of having a ready to go kill skill. You come right back with it ready to go and if you wait till your 30% or less on health to use it again, the process repeats, you will always have it ready. Pretty sweet and has been a lot of fun. Definetly need people to revive you though, I die a lot but like the "high risk high rewards" style of Krieg.