There is actually something of a gimmicky way to avoid reducing one of the skills. Put simply, you cannot reduce a Desynthesis skill that you have used within the past 3 Desynthesis actions.

So if you want to raise BSM and GSM, keeping LTW and reducing CUL, simply rotate your Desynthesis! Do a LTW, then do a GSM, then do a BSM, then do a LTW (to lock it back in place as untouchable). That will preserve (maybe even raise, if you do something high enough) your LTW, while raising both GSM and BSM. CUL wil be sacrificed if necessary.