There are six symbols in the game: → (Red symbol)
↓ (Blue symbol)
Λ (Green symbol)
V (Yellow symbol) ֍(Lightning - clears one symbol off each ghost on screen and used at the end of the final boss sequence) Note I can't find a Unicode character for lightning but you know what it looks like. ♥ (Heart - restores one heart)

Every level has a set of waves. The next wave appears when all the ghosts in the current wave are dead. However, some waves have timings so ghosts at a certain time - there's no way to speed this up. (a quite clear example of this is in Stage 2 before the boss) Therefore, you should focus on the last ghost(s) of a wave to get a good time. As general advice, you should just spam the symbol that comes next (you can look at this guide) and then read the rest.

The first ghost(s) in a wave is named Ghost 0.0s. If ghost(s) comes at a set time after these first ghost(s), it will be named after their respective times (e.g. Ghost 0.5s, Ghost 1.0s). These times are rounded to the nearest 1/10s due to awkward 30FPS recording. After you clear a wave, it takes 1.7±1.0s for the ghost(s) of the next wave to appear, whose level depends on certain stages, being hit by the last ghost, and if there is an upcoming boss.

I also recommend you play the game several times before reading this - that way you don't have to look at the reference every step of the way.

[Tutorial] Wave 1 Ghost 0.0s: →

Wave 2 Ghost 0.0s: ↓

Notes: ¤ Should be self-explanatory.

[Level 1] Wave 1 Ghost 0.0s: → Ghost 0.8s: ↓ Ghost 1.6s: → Strategy: →↓

Wave 2 Ghost 0.0s: ↓ Ghost 0.0s: → Ghost 0.0s: ↓ Strategy: →↓

Wave 3 Ghost 0.0s: Λ Ghost 0.4s: → Ghost 0.8s: V Ghost 1.2s: ↓ Strategy: Λ→V↓

Wave 4 Ghost 0.0s: →↓ Ghost 0.5s: ↓Λ→ Strategy: →↓Λ→

Wave 5 Ghost 0.0s: ↓ Ghost 0.0s: ↓ Ghost 0.3s: Λ Ghost 0.3s: Λ Ghost 0.7s: → Ghost 0.7s: → Ghost 1.0s: V Ghost 1.0s: V Strategy: ↓Λ→V

Wave 6 Same as Wave 5

Boss Stage 1: Λ→Λ↓ Λ→Λ↓ Stage 2: V→Λ→ V→Λ→ Stage 3: ↓→↓→→↓

Notes: • The music cuts out if you skip the cutscene too fast. (at least on my PC) • It takes 0.6s for the Level 1 boss to transition from one stage for the next. • The time it takes to draw the final symbol to first ghosts appearing in Stage 2 is 18.6s. • tSu_nAmi's mod note is no longer applicable now D: