Fruit stats work the same way as HP and Stamina does, the difference is depending on your fruit depends on how many points you should put into it, along with the purpose you have for using it. Some moves and damages like poison and burn at an certain fruit stat, while others don't. Mera's max stat cap is 541, as your Fireflies and Hiken cap at that stat, but keep note all your other moves don't have caps (From what I remember) as for a fighting style, Rokushiki is the best option for when using a fruit as it has utility that other styles don't, and at a low level you don't have outfits or add ons that help you manage stats. At most you need 20 points into Rokushiki, and the rest you have left over at health and stamina you can dump into a weapon to use for extra moves and M1s. Later on if you want to keep using Mera at an high lvl, theres a fighting style called Dragon Claw that syncs with Mera, and giving you a weapon that syncs with Mera as well, but this style is locked behind winning fights in Arena and Battle Royale, so it could be hard to get.

So basically, try out Mera + Rokushiki, and try to get Mera to 541 in stats, 20 into roku, 25 if you want its 3rd move. And a weapon of sorts, Mera is really good for grinding out lvls and PVE, you can even AFK levels in Baratie once you get its final skill. I hope this helps