Most skills in Summoners War can be improved by being leveled up, this is called a skillup. A skill can be leveled up by feeding a monster of the same family (awakened or not doesn't matter) to the monster whose skills you want to level up. The only exception are 2A monsters who need to be fed another 2A monster of the same family to get a skillup, so feeding a 2A Raoq to a 2A Kro will work, but feeding a 1A Raoq to a 2A Kro will not. Devilmon can skill up the skill of any monster. Since they are special members of their families, Fran and Eirgar can be skilled up by Fairies and Vampires, respectively.
Devilmon are one of the rarest resources in the games, you should only use it on nat5s.
In the following sections you can find detailed explanations on how the different types of skillups work.
Cooldown skillups are the simplest case, they do exactly what you think they do and they are the only skillups that show the effect directly on the skill description. For example a skill is "(Reusable in 4 turn(s))" unskilled and you skill it up, then every "Cooltime Turn -1" skillup will reduce the cooldown by one turn and the skill description will be updated to show the new cooldown: "(Reusable in 3 turn(s))".
Harmful Effect Rate Skillups¶
Harmfull effect rate skillups are additive, so if a skill has 50% activation rate for a debuff unskilled and skillups give 10% harmful effect rate then the maxed skill has an activation rate of
50% + 10% = 60%. This is why it's so important to max your Galleon's S3, the DEF break goes from 75% activation rate to 100%.
Activation rate is just the chance that a monster even tries to apply a harmful effect. If actually something happens is still dependent on other factors such as glancing, immunity, and the resistance check. See status effects for more info.
Recovery, Shield, and Attack Bar Recovery Skillups¶
These skillups are multiplicative and applied to the base healing/shielding/ATB boost amount.
For an example let's look at Colleen's S3: It says "Recovers the HP of all allies by 25% and increases their Attack Power for 3 turns. (Reusable in 5 turns)"
- Lv.2 Recovery +5%
- Lv.3 Recovery +10%
- Lv.4 Recovery +10%
are the skillups related to the healing. Max skilled she will heal for
(25 + 25 * (5% + 10% + 10%))% = 31.25%. The same also goes for shields and ATB boosts.
Your 2A Bernard will not boost by 45% if maxed but only 34.5%.
Damage skillups are special since they do not behave the same for all cases. The way skillups work, for damage skillups your damage will look like
ATK * multiplier * (1 + skillups)
for non-critical hits (and bombs), and
ATK * multiplier * (1 + skillups + CD)
for critical hits.