One thing they should point out is that you should use cards for the same character to do skill upgrades. To take the example in the picture, if you upgrade a Katyusha card with a Nina or Alina card, it will only give you 1/10 the skill points of a Katyusha card. A 1-star Katyusha card will give 100 skill points to another Katyusha card, but a 1-star Nina or Alina card will only give 10 points to a Katyusha card. The game also gives out a lot of generic skill upgrade Bokos that are worth 50 skill points, so it's probably better to use those if you don't have extra cards for the right character.
For higher quality cards (4-star or 5-star), if you get a duplicate card it is probably best to use it to limit-break one you already have, as long as you are below the maximum number of limit-breaks for that card.
SkillThis is strange!?...your skills aren't activating huh?Even though the other team is shouting out and powering up, Katyusha's team is remaining quiet!Katyusha referring to herself in 3rd person.Nonna:
If you're skill level is 0, it won't activate.Katyusha:
Skill level?......hm.
These students that I use for material will disappear huh.OrganizeTo make the skill level 1 or more, go to Organize→Skill→Strengthen and use local students as material for skill strengthening.Armor UP (large)It might be a little lonely with only 1 Katyusha.Nonna?I'm not talking about Katyusha right now you know!?Katyusha referring to herself in 3rd person.It's Skill Strengthening!!Sparkle
SparkleSparkleHere!Here!What are you saying, Katyusha?
You will always have Nonna.