This is a rather easy one once you get the hang of what the game uses for its calculation.
The truly easiest way to accomplish this is to spread it to all your party.
There is a total of 12 attributes to master. and you can have a party of 6 people. so using one person to master 2-3 of these will maximize your ability to accomplish this.
First off you need to know that different characters will gain different attribute experience. best way to understand this is there skill slots (non weapon) to decide on how they will increase. Best rule of thumb, the more slots for that attribute, the faster it will increase for that character. Using a party with the skill slots of 2 for each is ideal, but most likely you will not be able to accomplish that. (example being lightning for my party)
after you have decided on what party you will use, simply make sure that they use there respected skills, best to acquire this achievement when you plan on beating the boundary plain.
Best Char's for each stat:(non-hero)
General:*
Power:*
Combo:*
Special:*
Fire: Luana
Water: Yayoi, Plum, Reverie
Thunder:
Wind: Silvi
Earth: Sherufanir, Arbol, Plum
Dark:Zerva,
Holy: Ellis, Faina, Ryuryu
Extra: Vira-Lorr, Dyshana
* : Too many to list, since these are mostly the first two on every character, and yes even mages can level up there special attribute very fast.
Of course you could bread your hero any way you want, so he could fill the gaps you make based on your party selection. my party was...
Rex (bread for wind *Battlemage* with general starting at 9 and wind at 7)
Dyshana (for extra, and Dark, and Power *the use of ES attacks help with level ups as well*)
Ellis (Full Holy *Earth Nova is great for holy attribute*)
Plum (Water and Earth *great with Rog.3 equiped with +1 for each)
Luana (Fire *Mainly used her for stealing but realized she got another fire slot when I classed)
Beatrice (for Combo, Special, and Lightning *first ES is lighting*)
With that party I covered EVERY attribute, so the need of going back to level up one was nonexistent. I was able to get every attribute to 8+ and have not even passed lvl 300 in the boundary plain.
Also you can leach attribute exp with combo arts. Just an FYI, but some characters can not earn ANY attribute exp. Like Ellis cannot earn ANY Dark.
-Tip for skill equipment-
Best to do on a new game plus, but have your mages only equip magic, and fighters only equip Weapon skills (every attribute has physical attacks like *Ice Vein*) That way you will never have to increase Str for your mages EVER.
FYI:
Attribute exp is gained based on AP used, effected by level gap (attribute level, monster level, and your level), and finally effected by hit amount (must land for exp gain). After that gets calculated it gets multiplied by the character gain ratio (like I said above, pick a character that has a high gain ratio, and not one that has a 0)
*edit*
After much testing I found that you get faster exp for your attributes for a higher level char. so, it doesn't matter what the monster level is, since each monster gives a set amount of base attribute exp. The higher level, and status (such as boss) yield higher base exp. Still not sure if exp is no longer added if you attack after overkill, or if it stops after break, but one of those DOES happen. (hence a char I used last in all battle got 0 exp)
Also pure combos are best for level up (Crimson Edge + Fireball + Fire = Explosion) for a fire pure combo