The big thing about Elements is that they are one per battle deals so you need to pay attention to the field affect board. You don't want to use a White element when you have two black elements on the field as it weakens the damage and after the element is used it's pretty much unusable until the next battle. The only exception are the Capsule/Item Elements which can be used multiple times depending how many you have in a set.

How many element slots you get is determined by character and which spells you can use are ranked so for instance most character will eventually get eight Lv. 1 slots to use low level magic but mage characters tend to have reversed slots where they have few low level slots but tend to be able to cast several high level elements. Low level slots are also the only way a character can use elements of their opposing element, as they won't get access to high end versions of spells. For instance, Serge can't use high level Black Elements but has easy access to high end White Elements.

You can also put a low level spell into a high tier slot and increase its power which is usually shown with a +1 or higher number so don't feel obligated to throw out low level Elements.

I know this all sounds complicated but honestly, much like Junctioning, it's pretty simple in practice and you can easily get away with letting the computer set up your elements for characters and be fine. The only times you need to be careful with elements is if you decide to bother with Summons and for the final boss battle in which case you may have to do some set up to make it happen.

Every character gets three unique skills which are either really powerful attacks, special buffs or rare abilities like one characters power to transform into monsters you've defeated or Kid and Fargo's steal abilities. They are located in the characters third, fifth, and sevenths tiers and once obtained can't be removed. The first two are obtained by just progressing through the game, but several characters need to do special side missions or find their third techs in treasure chests.

There are dual and triple techs but painfully few considering how large the cast is. There are a total of nine dual techs and two triple techs. Interesting enough these techs tend to be different elements than their two host use, for instance Serge is White Elements, Glenn is Green Elements but their Dual tech will be Red so this does give you some options in battle though you may not need it since these abilities are so rare.