There are 99 levels in this game. To gain a level your character always needs 100 experience points. However, the amount of experience you gain per action depends on the level of the unit you do the action on.

The thunder spirit (Gote) has three skills that will increase the amount of experience your character gets. (+10%, +20%, +30%)

The maximum amount of experience your unit can gain per action is 50 points. So it takes a minimum of two actions to level a character up. The goal here is to teach you tricks to only need two actions to level a unit up.

1 - Early on you simply want to level your fist few levels on the first floor of the tower of trials. For this I would only take a few steps forward and let the enemy close most of the gap for you. On easy level this isn't such a big deal because your fallen character come back to life after the end of each battle.

Note that I would strongly recommend building up Gote and Kashis spirits first. Gote gives you both a powerful bow and a morning star as well as experience boosts to help your characters rush through enemies. Most importantly about gote is the boost in hit rate which is critical for attacking your own and higher level units. Kashis gives you a high hit high evade dagger and +10/20/30% Devotion skills that will help increase the rate you build you spirits up.

2 - Once you have a few level 3-5 characters you can skip to phase two. Here you will be hitting your own characters to gain experience untill the enemy gets closer and then killing off all but one of them. Additionally you can use the 'shoot' command to push the enemy back in order keep them away from the units that are fighting each-other. Tell a unit to attack and then hit the 'X' button. Now aim for the red section to push the enemy back.

Action bar and critical or weak hits - Where on the gauge you initiate an action determines how much damage it will do. When you want to kill a unit you should go for a critical hit, located on the right side of the bar in the blue zone. When you are hitting your own units for experience you want to aim for the very early section of the gauge to do the least amount of damage. The following is a lame representation of the gauge.

First-half Second-half

First-half is used if you don't want to kill what you are hitting.
Second-half is used if you want to kill off what you are hitting.
The underlined portion of the second half is the critical zone. You will do much more damage in the critical zone. You can also gain an ability from the spirit Amu called arms master. This ability makes the entire second half of your gauge a critical zone.

there is another trick which you are mising that extremely helps leveling up. take a basic lv.1 cure and engrave it with white and minstrel seals to make it a 25 area and get the coin cost down to ten and the amount to use as high as possible (80) the have caracters that are of higher levels than then the guy you are trying to level up. beat up on each other and then use the coinfiegn on two or more of them. this will usually result in 100exp if used on any character two or more levels higher and the more people you ue it on the closer to 100 dev points.
quick way to catch characters up with each other and if you use this tactic in weak trial towers, you can get your characters up to high levels without worrying about the enemy killing you off. also, using Re Vin on any character two or more levels higher than you gives that character 100 exp.

The max experience per hit is 50.

So yes hitting two or more higher level characters can result in 100 exp and dp. You can buy a cure coin from the shop that hits four squares.

I haven't lost any characters so I didn't know about the revin experience.

Getting the coin cost down only serves to reduce the number of turns it takes to level up your character. Still you have to increase their life points to get credit for curing them and if they are leveling up at the same time by hitting eachother, it will help your lower level character keep getting 100 of both for longer.

Leveling up isn't as hard to do as maxing all the spirits. If you get a level 99 character at the beginning of the game and then use that chartacter to both level up and gain dp you can gain skills to help your characters gain dp faster once they are 99. Look in my spirit guide FAQ and unlock the holy spirit as soon as possible with these new characters.