The heart of Skies of Arcadia Legends' battle system was the Spirit gauge: a single meter that fueled the attacks of all four party members. Both magic spells and individual character's super moves use up set amounts of Spirit points. However, each character also automatically restores a certain number of Spirit points to the gauge each turn. Furthermore, a character can use the "Focus" command to double their Spirit contribution for one turn. The result of this system is that every battle in the game comes down to careful management of character actions and the Spirit gauge. So developing sophisticated strategies that involve all four characters is a must.
Of course, this system is at its strongest during difficult boss battles. It usually is not necessary to worry too much about normal enemies, particularly since the system lets the player annihilate most random encounters by using Aika's all-enemy attacks without cost every turn once the party generates enough Spirit. While the game leans towards the easy side, there is a category of enemy that always proves to be a challenge: the optional Bounties. All of the optional Bounty enemies, as well as the Pirate Hunter Piastol, increase in strength to match the party level of the heroes, so it is impossible to beat them easily by taking the time to level up. As a result, the fights against the Bounties and Piastol are some of the most strategically demanding and fun that I have ever seen in an RPG.
The biggest problem of fighting off these enemies is that their offense is often enough to wipe out the entire party in very short order. However, one of the greatest strength of the Skies pf Arcadia battle system is its strong support for defensive strategies. Every character has at least one defensive/support special move that has turn priority, in other words always goes at the beginning of the turn. For example, Vyse has Skull Shield, which shields the entire party from basic physical attacks (and follows up with counter-attacks) for the entire turn. Aika has Delta Shield, which makes the entire party immune to magic for one turn. Fina has numerous powerful healing moves and an all enemy debuff attack called Lunar Winds. If the party is generating enough Spirit, Vyse and Aika can use both of their defense moves to make the entire party immune to regular attacks and magic spells (though enemy special attacks, which most optional bosses have, are unaffected).
So, some fairly complex and versatile strategies are possible for example, a typical strategy might be:
Vyse- using his best attack Special
Aika- using items to heal/buff
Fina- Focusing to build Spirit
Enrique- using Justice Shield to halve all enemy damage
Alternatively, if fighting enemies without Special attacks:
Vyse- Using Skull Shield
Aika- Using Delta Shield
Fina- Alternating between Focus and using items
Drachma- Alternating between Spirit Charge and attack Specials
Simply attacking every turn with every character and healing as necessary won't cut it against many of the super-powerful optional bosses. The game also opens up more strategies by making a full-party super attack if the Spirit meter caps out. So the player can choose between an attack strategy and turtling until the super attack becomes available.
I find this kind of full-party strategic combat to be much more interesting than the somewhat more common "save MP until boss fights, then nuke the enemy with your best spells every round" battle-systems. The only real weakness of the Skies of Arcadia system is that magic is useless in it. Most spells only do the same kind of damage as special attacks, or mimic the effects of items. However, Special attacks can do any kind of elemental damage, and items do not use up Spirit. So healing and buffing spells are completely over-shadowed by items, and attack magic just doesn't have much in its favor.
Still despite that one weakness, Skies of Arcadia has one of the best RPG combat systems I can remember. The only way I can think of improving it would be to couple it with the Lunar series system of enemies broadcasting their attacks, so that the player can devise counter strategies.