Warning: I talk a lot. 
INTRO
Hi, I have experience providing vocal synth producers with original Japanese lyrics by me or converting their English songs into Japanese versions. I've been "studying" Japanese lyrics ever since I decided to study Japanese about a decade ago (started with nursery rhymes while learning hiragana as a teenage newbie, got into visual kei in college while working on passing JLPT levels, then ended up in the vocal synth fandom full grown).
I am not sure about your Japanese skill level, so I'll just write this message toward newbies.
I really recommend keeping a folder of your favorite Japanese songs (on the computer and printed out so you can read it throughout the day). It used to be difficult to memorize lyrics. I would pick one song to study for about 5 days straight and would spend about 20 minutes listening to a song over and over and singing along with the words until I didn't mess up (you don't have to be good at singing). I think I have like 80+ songs memorized in Japanese at this point.
Eventually, this will help build a library in your head of common song vocabulary (ex: moon, love, searching, disappear, shining, sky, voice, etc) and useful grammar. I also realized my own preference for lyrical content: I prefer songs that talk about nature (sun, rain, moon, wind, darkness, etc) and I love lyrics with chilling irony, religious allegory, and sensualized afterlife in both English and Japanese. (Basically, I’m pretentious.
)
LITTLE BUSTERS!
Before I continue, I want to help analyze “Little Busters!” by Rita since you brought it up.
Note: You accidentally spelled へ as え (pronounced identical when used as a particle in grammar, both spelled as “e” in English only as a particle). And it’s not translated as “flying” or “raising” because those would be infinitive verbs (飛ぶ、上げる) or continuous form (飛んでいる、上げている) depending on the English translation. They’re actually imperative command form.
高く飛べ 高く空へ 高く蹴れ
Takaku (adverb) tobe (imperative verb) / Takaku (adverb) sora (noun) e (destination particle) / Takaku (adverb) kere (imperative verb)
Fly high, High toward the sky, Kick high
高く声を上げ
Takaku (adverb) koe (noun) o (object particle) age (imperative verb)
Raise your voice high
TO ANSWER YOUR QUESTION
Dropping the “u” in “ku” is called
devoicing. You hear this in “desu” being pronounced “des”. It’s used for a punchy rhythm. If the “u” was kept, it would make it harder to pronounce quickly.
SIMILAR EXAMPLE
I can only think of one other song that uses this technique, it’s metal visual kei genre, though (sorry if you don’t enjoy spooky leather-clad femboys with a god complex like I do). It becomes clean that dropping the "u" makes it more "aggressive". The lyrics are also not beginner friendly at all. I will explain them despite no one asking me to because I am bored and like to ramble.
MORRIGAN – THE DAWN DEEP DOPE
YouTube link (not timestamped because I don't want to scare people with unhinged metal vocals)
0:21 (Warning: Unhinged metal vocals at this timestamp)
THE DAWN DEEP DARK漆黒感情奈落剥奪人形（ドール）
THE DAWN DEEP DARK shikkoku kanjou naraku hakudatsu DOLL (all are nouns, DOLL is gikun pronunciation)
Pronounced ZA DOON DIIP DAAK
[shikkok] kanjou
[narak hak-dats] DOORU
Literal translation: THE DAWN DEEP DARK Jet-black emotions the abyss deprived doll (series of spooky concepts and lacks any particles or grammar)
My interpretation (when it doubt read the sentence backwards): THE DAWN IN THE DEEP DARK A doll deprived in the abyss of hopelessly dark emotions
0:48 (Warning: Unhinged metal vocals at this timestamp, too lol)
THE DAWN DEEP DARK純白腐敗現実救済
THE DAWN DEEP DARK junpaku fuhai genjitsu kyuusai (all nouns with no particles/grammar)
Pronounced ZA DOON DIIP DAAK
[junpak] fuhai genjitsu kyuusai
Literal translation: THE DAWN DEEP DARK pure white decay reality salvation
My interpretation: THE DAWN IN THE DEEP DARK Salvation from the reality of pure-white decay
LYRIC WRITING TIPS
I will illustrate helpful “rules” I keep in mind while translating. Try to pay attention to these forms in your own favorite songs!
NO VERB? NO PROBLEM!
A lot of beginners writing lyrics in Japanese for the first time think you have to end every sentence with a verb. You don’t have to. I will show some alternatives.
Rose Noire - FEED
(Timestamped, don't worry, it's just goth violin music, no agonized screaming present)
This song is so beautiful in Japanese, it’s frustrating I can’t think of a way to convey it that compares in English.
04:22
世界に光を 僕にくちづけを
Sekai ni hikari o (a verb would go here but it’s implied) / Boku ni kuchiduke o
Light to the world / A kiss to me
君には大陽 海に月を
Kimi ni wa taiyou / Umi ni tsuki o
The sun for you / The moon for the sea
星屑つむいで 僕は夢見る
Hoshi-kuzu tsumuide / Boku wa yume miru
Spinning stardust and / I dream
息を止めたままで
Iki o tometa mama de
With my breath stopped
The last line was shocking because the previous iteration of this chorus ended on 泥の眠りの中 (Doro no nemuri no naka / Within a muddy sleep) and all of a sudden it dropped a bomb on us that he was either holding his breath or DEAD!!! (And it’s worded beautifully!)
For the sentences that end on を, you could have verbs like 捧げる (offer) or 与える (give). But why would us pretentious people want that? lol It’s more poetic without. And saves precious available melody notes.
REARRANGE + CHANGE VERBS
If the available MIDI notes give you problems, rearrange the words!
Don’t ALWAYS use infinitive form/past tense, use ～て and ～ます stem to connect lines (they do the same thing, ~masu just sounds more poetic).
Rurutia - Selenite (セレナイト)
(Timestamped, this is the safest song. Just a slow, calming but sad piano pop song.
) I made the translation more literal here so it's easier to understand.
0:28
ああ 揺らめく灯火 瓦礫の街に
Aa / Yurameku tomoshibi / Gareki no machi ni
Ah / A flickering flame / In a city of rubble
Boring way to write this same line: 瓦礫の町に灯火が揺らめく
Putting a dictionary or past tense form of a verb in front of a noun turns the verb into an adjective.
鳴り響く 透明な調べは願いを湛え
Nari-hibiku / Toumei-na shirabe wa negai o tatae <--ends on ~masu stem, could use 讃えて but it wastes precious melody notes and doesn't sound as fancy
Resounds--The clear melody filled with wishes
Shirabe is a fancy way to say 曲 or メロディ.
今 焼け落ちた空は 水の底へと沈んでいく
Ima / Yake-ochita sora wa / Mizu no soko e to shizunde-iku (iku is pronounced yuku here because it is poetic and archaic)
Now / The sky that burned down is / Going to sink to the bottom of the water
満ちていく月明かり ああ
Michite-iku (yuku) tsuki-akari / Aa
The moonlight is going to wax / Ah
As the moonlight gradually fills the night, ah...
Could say 月明りが満ちていく if you want to end on a verb instead of using it as an adjective.
青く燃える夜が君を染める 儚いほど
Aoku moeru yoru ga kimi o someru / Hakanai hodo
The burning blue night paints you / To the extent of being transient
細い肩をきつく抱いていても
Hosoi kata o kitsuku daite-ite mo
Even if I hold your slender shoulders tightly
Boring way to write this line: 私が君の細い肩をきつく抱いていても
君は消えてしまいそうで
Kimi wa kiete-shimai-sou de
You seem as though you might vanish (消える is also a euphemism for death in Japanese).
LITTLE BUSTERS! Part 2: Electric Boogaloo
When I make lyrics for vocal synth producers, it takes a lot of thought. You have to count available MIDI notes and rearrange your lines based on that.
So for your example of Little Busters!, you could rewrite the line in a bunch of different ways but it all comes down to catchiness and available melody notes.
Horrible way you could rewrite the lyrics (adding pronoun when the verb already tells us who is supposed to do the action without it) aka the “I used Google Translate” method:
君は高く飛べ （君は）空へ高く飛べ
（君は）高く蹴れ （君は）高く声を上げろ
Even more horrible way (same meaning but instead of an order, it is a request):
君は高く飛んでください （君は）空へ高く飛んでください
（君は）高く蹴ってください （君は）高く声を上げてください
More creatively, you could rearrange the words depending on what best matches the BGM. Ex: 高く空へ
The end, I dunno what else to say.
