Best way to create CE bomb for 5* FLB CE's?

Yes.
You'll need 1* and 2* CEs for this, any other CE of any other rarity you don't want, you can feed to them. Here are some tips (i personally use):

What you're ultimately trying to do is make a massive ~2kk EXP CE bomb. This takes a bit of time and effort and you're actually going to split this task to more, less massive EXP bombs of 100k and 18k first.

1) The first task is to create 18k EXP bombs:
2* CE can take 56k EXP max on level 15 (base level). This is perfect since with 18k EXP you're trying to feed it, even if you get super success (2x EXP) or great success (3x EXP), you're ending up with 36-54k EXP, which leaves you just 2k EXP shy of maxing it out. If you get SS or GS, this CE is worth as 2-3 18k CEs in one (duh).

I usually try not to mix same CEs when i feed EXP to a 2* CE, in order not to purposely increase their level past 15 (i don't attempt to LB them). You can do this if you really need space or don't have any other CEs but respect the base level curve because the more levels a CE has and the higher level it is, more and more QP is required to level it further. The point of this guide is to save you QP as much as possible and that's why you work with base levels of low rarity CEs.

2) The second task is to created 100k EXP bombs:
The best CEs for this job are 1* CEs on level 20 max (one time LB'd) or 2* CEs on level 25 max (one time LB'd). I do this with 1* CEs since they are more easier to get but if you lack them, feel free to use 2* CEs.

You make a 100k EXP bomb by feeding 5 18k mini EXP bombs to it. That's 90k EXP + EXP these base CEs are worth (~100k+ in total). LVL 20 1* CE can hold max 133k EXP while LVL 25 2* CE can hold 260k EXP. You don't have to worry about super/great success mechanic at this point because even if it does happen, it only doubles STD EXP, not ACC EXP (STD is standard EXP CE is worth, ACC. EXP is accumulated EXP through past feedings).

3) The third task is to create and prepare a 2kk EXP bomb base.
You do this by max LB'ing a 1* CE to level 50. Level 50 CE can hold a max of 2,082,500 EXP, which means you need 20 100k EXP bombs to feed it. Try to do this in one go because as i've said, if you fragment this task, it's going to cost you a lot of QP. You want the CE to be on the lowest level possible so that it's cheaper to level in one go.

4) Stock up on these 2kk EXP bombs and put them in the storage and their 100k pieces.

3* CE's (max LB'd to level 60) max hold is 3,599,000 EXP.
4* CE's (max LB'd to level 80) max hold is 8,532,000 EXP.
5* CE's (max LB'd to level 100) max hold is 16,665,000 EXP.

5* CE is going to need 8 of them at least.
You reach other values by mixing 2kk EXP bombs with lesser ones (you can make 1kk bombs as well, if you need/want to level 3 star CEs).

How much QP do you save leveling a 5* CE this way? Seems like an awful lot of work especially since managing all this on a phone is really tedious.