While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
How to get more Souls to limit break?
1. Soul Quest
This is your one-stop shop for souls and can be done three times a day, at least. Note that collaboration characters and non-KOF characters will leave the soul quest after the campaign has ended.
2. Events and Festivals
If you go to the battle section then to the event quest you'll see that there is a special quest with a scary looking boss. It actually changes every week and the rewards are colour souls, different per difficulty and the colour rotates every week. There are also characters souls to win. For example Alexandrite hard difficulty rewards you with 97 Robert souls*. Expert level difficulty reward is colour souls of a particular colour so you can use it for any of your matching color fighter. Each difficulty is limited to 3 a day.
Then there are other events and festivals that will award you souls, check the rewards! Check the festival section for more information.
*This was changed in later revisions to colour souls but may change again.
Epic quest is the only way to get souls for the epic quest characters.
To get bronze souls you'll have to complete the festivals and the lowest difficulty of the advent bosses.
3. Story Mode
Look at the images through story mode. The portraits will have a soul icon with 3/3 because you can only do it 3 times a day. For example, King’s Soul Event can be found in KOF95 Stage on NORMAL level. That makes it relatively easy to level her up, whereas Mai 95’s soul event is on KOF98 EXPERT level, so you’ll need to do some harder fighting to get there.
4. Secret Shop
I wouldn't do this but if you got rubies to spare, go for it! The shop changes stock every four hours. You'll usually see 3 colour souls for sale for 100 but if you're lucky you'll see 10 for 300 rubies.
5. Exchange Shop
Go to the shop and exchange soul dust for souls. You'll need 3000 soul dust for 10 souls. The tactic here is to salvage the most common characters into dust because you'll get them again. Or you just don't care for them. You have been building soul dust collection with every roll. You can see who is the most common for you because they will have a high number out of 600.
Before you burn your collection to dust, you are limited to collecting an amount per day! AND the shop doesn't have souls for everyone. Remember that you get a stat bonus for levelling every character… choose wisely!
Limit break will use the souls first then the next item in the row.
Raising this level will give you souls, look at the reward list. However this uses an item of food that is liked by another character. Getting these items is dependent on rarity from story mode quests. Getting to level 3 is easy, reaching level 10 took 300 quests to get sufficient drops! It’s not really worth it. A better option is to use the special items from quests such as the photographs, which is worth many more points.