I still think L/F is kind of a weird idea for an endgame class but I haven't tried it so I'm not really knocking it. L is pretty durable on its own and a L/F will do more damage than a F main if you want it to, but really the thing to think about is what's the L/F's role in the party?
With a squishy N/B up front, the L is going to have a tougher time keeping her alive with only half-powered F skills, and will be doing subpar damage by having to spend turns on protection skills. Doing high link damage takes a few turns of setup and having to do anything else delays your offense, causing attrition damage to your party. Not to mention an A/M will have a relatively (slightly) harder time healing everyone than an M/A, but with a dancer you should be fine because waltzes are potent.
Subbing Bushi on your NS will help somewhat because you can survive a fatal blow once per battle, but if any class in the game needs a babysitter, it's N/B lol. When you're in boss battles it's like... your L/F can't do two things at once, it's always a choice between doing more damage and taking more damage at the same time, or doing less damage while reducing the damage your team takes. At level 99, farming almost anything except Warped Savior is really a pushover, so yeah a F main may not be contributing much at that point, but on the other hand it's like... your N/B and R/B will do so much damage they don't really need any help killing most things, so just let the F stay an F and do what it does best: keep everyone else safe while they beat the crap out of things.
I guess you could spec a L/F to be a tank but tank skills get expensive to use and without F's passive to restore TP when getting attacked, I think it'll be tough. You won't want to ever use Vanguard when trying to tank, meaning good link damage will be harder to get (both because the L's power itself will be lower but also because your N/B will always act first and your D might also act before the L/F can, leaving only R's attacks to trigger a portion of the maximum potential links).
I'm curious to know how it works out for you. Honestly I think between a D/N, N/B, and R/B you will have plenty of damage to make a L/F's damage unnecessary. I guess as long as you can weather the powerful attacks from bosses like Warped Savior and Fallen One, then that's all that matters.
You could start from a build like this which uses 86 skill points (out of 116 possible for a level 99 who was a retired at 99)
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So that leaves you with 30 points to invest in damage skills (this skill set includes Improved Link + Link Mastery so you could invest in 1 link, probably Lightning)
Kinda splitting hairs here I guess since EOIV isn't really that difficult aside from Warped Savior. It's just that starting earlier in the series trained me to utilize the power of a balanced party so I'm a proponent of that style.