I'll try to explain some skills.
For the attack support Z skills it's easy to see the effect just equipping a summon and shooting. Notable examples I used were ice attribute double bullets (early game from the inn version of big boobed blue magic girl, later many other enemies have it too) and shadow attribute matrix type attack costing 2hp (from the flying girl you use to show all these skills). If you combine it with skill lowering cost by 1 hp - the last skill on the attached pic from previous post, coming from killing extra bosses on a book, you can have a very nice skill for 1hp. Unfortunately you can't lower cost 1hp skills to 0hp.
I'm not a big fan of X skills, these mostly require too much positioning and are not very useful imo. Harpy dive might be an exception for its long range.
Attack change skills - again equipping these skills and trying effects should be easy. Just remember to equip skills that are shown in blue color meaning it's for the right girl form. I like the frog attack for Moon and icicle instead of bomb in air for Pure. Both come from big boob blue magic girl early game. Once you get red sword forged from red wing these change skills become obsolete.
[FA] skills - meaning final attack, these skills trigger when the summon's Hp goes down to 0. Not very useful as you basically don't want to go that far and healing skills that keep your summons high on hp are easy to obtain. And you can use Summoner's R magic to heal or even revive 0hp summons if you are far from base or bath/inn.
Other change skills - you have Moon's dash skills explained, the other two add small bullet immunity, "ウォーター・グレイズ" - water grace (underwater) and "ダッシュ・グレイズ" - dash grace (during slide). Note that the last girl form you get, devilish princess, also known as Devilish has intrinsic small bullet immunity while dashing in the air.
Dive skills - these are actually water breathing skills, reducing air usage while in the water to 80% or 50%. They don't stack, so use only better version coming from the stronger ballet girl. You will still need grey bottles from the shop that restore air to reach some underwater areas. Jetpack for MC girl adds in air vertical dash, the last skill adds 1 hit to Moon's C+Z charged fireball, stackable up to 3 hits added to base 5 hits, so max 8 total.
Healing skills - there was comparison earlier in this thread. As far as I have tested healing skills don't stack on one summon, so choose the best one for each summon. The best combo I have found is to use 2 summons, each having 1 healing skills and 1 special skill improving trigger chance, best healing is "High pixie heal" from dark pixie ハイピクシーヒール, which heals both summons twice a second with a 1/3 chance, but its available only once you get to devil tower, in middle game you can use standard "Pixie heal" ピクシーヒール from blue pixie which is the same but only once per second. Second skill to use is a skill to improve the chance to 100% found on blue pixie. You can have 4hp per sec recovery for both summons with this combination and this is enough for a summon to use a good Z support attack a lot and still be full on hp. Two such summons and you have bullet spam capable of quickly clearing almost anything.
Rescue skills: 1. when falling from high places, 2. faster shake off H with <- -> 3. same as 1. at least they have the same description, one of them may be bugged 4. when drowning. I'm pretty sure there is also a rescue skill missing here for being squished by moving blocks.
Elemental protection skills - these are stackable, in order from pic: 1. shock tolerance 2. shock immunity 3. ice tolerance 4. ice immunity 5. fire tolerance. There is also fire immunity, missing here, which I mentioned earlier in this thread. Tolerance gives you 100% breath protection, 75% small bullet protection and 50% large bullet protection for a given element, immunity gives you 100% protection to all type of attacks with that element.
Protection skills: 1. Crouch to get protection from rush attacks (not working against swords) 2. High chance of evading breath attacks (just get all ememental immunity instead). 3. Fire and freeze resistance (same as 2, get immunity instead) 4. Mental shield - can transform to mc girl on high excitement, without this skill first red girl boss battle in early game might be confusing (why is CC not working?). 5. Poison immunity
Sight/Gather/Analyze skills - first three let you respectively - 1. see invisible blocks that normally only appear when you touch them; 2. choose the type of naked minigame or avoid it to save tickets when caught by crouching dragon girl in areas with her statue; 3. see wind direction in the area after the bridge destruction battle, this is the same as green item you find in that area, but can be used earlier as twins in the same area provide this skill. Analyze skills let you 1. see element and type of bullet you get hit with; 2. see damage given to enemies in numbers, same as item from the shop later in the game; 3. see basic information about enemy you are fighting with, like their type, resistances and vulnerabilities.
Crossed swords skills increase damage - each one works for different type enemies, usually covering very few of them, also most of them drain 1hp from summon every time they trigger. In order from the pic with jp names shown on analyze 1. 毒虫 poisonous worms (slimes), no cost 2. ハーピー harpies 3. ゴースト ghosts 4. 歩兵 infantry 5. hp>48 enemies, cost = 5hp 6. 肉壁 meat wall (golden girls in the area where you unlock Moon form). Last two skills work only on flying summons and they make summon position in front of mc, first one during Z attack, second one always.
Exlamation mark icon skills (not to be confused with [!] skills) either add or remove special movement modifiers, Pure starts with plump body attribute, meaning she has inertia while running, this can be either removed (harpy skill) or put on another form (girl lying on her huge boobs skill). Moon starts with edge standing skill and this too can be removed or put on another form. Skills in order from the pic are: 1. remove plump body 2. add plump body 3. remove edge standing 4. add edge standing. Both these attributes can be combined to achieve [MIX] skill. Last two skills in this set without icon are: 1. Change chance on pixie healing skills that have 1/3 chance to 100% and 2. Divine protection - small bullet and arrow immunity while not moving.
H skills - 1. X-ray (press up arrow during H) 2. and 3. add many bonuses, not sure which one is from the 1st minigame or from the 4th minigame. The one from 4th minigame is obviously better. They change naked posture, moving speed, fap speed and cum regen speed. 4. Weakest of them all, that can be found on summons, adds only fap possibility and some summons may react to hero H with their own masturbation.
Special skills: 1. 1/4 chance of not using HP for Z attacks on soul type summons 2. Increase number of hits of X support magic attacks (with round icon) at the cost of increased hp consumption 3. Decrease hp consumption by 1 for Z support skills (but not down to 0).
