Bathtub installation in mortar bed?

I had a company install my bathtub and do some tile work. Per our contract, they were to install the tub on a mortar bed, and the tub was to be level (obviously). They did neither. The level isn't off enough to be of great concern, but the way the tub was installed, the feet of the tub (or at leas some of them) aren't even touching the floor. This is visibile through the plumbing access -- else we would never have known. Long story short, we were able to get a partial refund for the shoddy job, and are attempting to fix this ourselves. Rather than remove the unsupported tub (and all that that would involve) we are taking out part of the wall that's behind the tub (that room hasn't been finished yet anyway) and are going to attempt to push mud under from the back side -- there's not enough room through the access. I say all that to ask this question: What is the recommended/best mud/mortar/cement/whatever to use in this situation? Obviously we want something that won't set up on us too quickly -- while we are trying to get everything situated in this awkward scenario. Thoughts/answers appreciated.