If you are serious about wanting a level floor and a decent tile installation you would pour a self leveling compound. You could level the floor using a compound suitable for wood underlayment and then screw down the backerboard (using extra-long screws, if necessary), or you could screw down the backerboard directly and then use the self leveling compound on top of that. Here is one type of self-leveling compound:
Self-leveling compound

You would have to add temporary "dams around the perimeter to keep the compound restricted to the area to be leveled.

You could also cut a bunch of wood shims and try to get the cement board level that way, but it would be a lot of work and might not lead to as good a result as you need for tile setting.

P.S.: I would also talk with the RediSet people about that shower pan leveling job. Having one shim supporting the corner only might allow cracking or movement of the pan. This installation might be helped if you used self leveling compound that ran under and up to the lower edge of the pan and supported it all the way across. The proper way to have done this would have been to first level the whole floor, including the shower pan area, so no shimming of the pan was needed.

So I can use that instead of using backer board?

Yes, in the right circumstances. I just had a retiling job done on an unlevel concrete floor and the self-leveling compound made it flat as a pancake and plumb. There are just some things you need to know in order to determine if your existing floor without backerboard would work:
Consideratons

In particular, be aware that you might have to staple metal lath screen to the plywood first and have a minimum thickness of maybe half an inch of compound. It might be easier to screw down backerboard first and then apply the compound directly over this. If you go with compound, research it by carefully reading some spec sheets as various compounds have various requirements.

Thanks for the info. Luckily the area to contend with is only 72"x52" - the rest of the bathroom outside of the shower/toilet area is fine.

My bathroom is 11'x21' - lord knows, I have many other spots I can tile first. ;)

You need the floor flat - no necessarily level.

Cut those shims off - that are sticking out from the pan. You need the cement board to have full contact on the sub floor. Spread thinset on the subfloor and attach 1/4" cement board with screws. The thinset will ensure no hollow spots. Then pour SLC to make the floor flat.

Go to the website for whatever cement board you are going to use, they will have instructions about how to attach it to the subfloor.

Tell u what, maybe go to the tile redi site and read the PDF on installation for the pan.

"Please note, do not place the mortar directly on a wood substrate. First, waterproof the wood substrate with a waterproofing membrane, liner, or sheet rock so the wood does not absorb the water from the mortar."

That shimmed side looks about 3/4"? if so and that is a white thinset mortar its beyond the ability to use thinset that thick even a medium bed it could crack loose with human weight and deflection of ply being the concern....

In regrads to SLC you need to look at deflection rating for your specific joist span size and then assess the subfloor before jumping in that boat....

We are in the midst of a master bath reno. We are using 12"x24" rectified tile and wanted 1/16 " grout lines. The bathroom floor had some low spots and a slight slope. We had learned from using large format tile in the major part of the house that you are not able to "work" the larger tile as easily to get it flat and have no lippage or ridges. We ended up pouring a self leveling compound as someone suggested. Just a word of warning about doing it - besides the suggestions already given, be aware you have to mix it up and get it poured in 20 minutes. We had 2- 50 lb bags to mix and it has to be mixed for 2 minutes as you pour the compound in the water. Mixed with a paddle on a drill. Nearly burnt up our drill doing so. Luckily a friend just happened to stop by and he was able to pour it while hubs mixed. I did the timing. We had two heavy buckets to carry it and get poured. Hubs used our carpet rake to move it around the floor but it wasn't as easy as it looked. You would think it would run the direction it needed to but it starts to set up quickly so be prepared. Hubs said he should have made himself a tool that he could have pulled across the width of the bathroom and then one for the length. It certainly helped to get our floor level but we still had two smaller dips which he then filled in with some other material. Our floor is in now and looks beautiful. Just make sure you have enough help so that you can work quickly. I would have never been able to pour the 50 lb bag or carry one of the buckets from outside into the house.

SLC in bags is typically mixed up in a trash can very close to where it's being poured.