Okay, before we start, you have to know this, cause it makes the game A LOT easier:

You move/swim faster, whenever you're not shooting!



Expert Mode will be unlocked upon beating the normal mode for the first time.

It's not really that much harder than normal mode, though. Enemies don't have more health and the projectiles are just barely harder to dodge. The real gimmick is, that normal insects don't drop hearts anymore, except for the rare dragonfly. But, you'll receive a guaranteed heart upon boss purifications this time around, so it's kind of a fair trade of.
Expert mode is "Night mode" though, so that's nice.

If you struggle against the bosses, this may help you:

2nd boss:
If he shoots his bullets, the middle one will always aim straight at you. You can easily dodge it, if you have just a tiny bit of space directly above or beneath you. Which means, the only instances of danger occur, whenever there's a bomb at the middle locations. If that happens, stop shooting and use your increased speed to get around those corners and into safety again. You should be able to make it without any problems, if he hasn't started his bullet shooting pre-animation by the time you start your maneuver.

3rd / Final boss:
Try to remain at the lower left corner of the area. If he uses his diagonal tongue attack, he'll always start on the upper side, so you'll have enough time to react. Note, that you're actually safe beneath the tongue, if you go closer to his arm.
And if the tadpoles on his arms start to shoot their aimed bullets at you, just move up while keeping the space bar pressed and by the time you hit the upper limit of the area, the 4 waves of attacks will have ended.





I don't know the exact time required to do this, but the general consensus seems to be, that a time below 8:30 will do the trick.
Note, that there seems to be a bug, which prevents this from unlocking, if you haven't beaten expert mode, yet. So consider doing the achievement above first.

The 4 heart form is NOT required to do this. You can easily beat 8:30 in the 3 heart form.
The 4 heart form absolutely shreds through the final boss, though.

Fun fact: The speedrun record as of publishing this guide is 6:44.
And here it is, performed and uploaded by Lionboy730:

It's highly adviced to do this on normal mode. You'll be force fed 2 hearts in expert mode, and surviving the entire 3rd part of the game without taking damage in the bulky 4 heart form would be unnecessarily difficult.
But grabing 1 heart in normal mode for the fire rate upgrade is absolutely worth it.

If you DO take damage, you can reset the game by keeping the [R] button pressed for 4 seconds. This will skip both the need to be damaged another time and the game over -> try again screen.



Do this one last; I consider it to be the hardest out of the 4 achievements.
Note though, that you ARE allowed to take damage once.


Don't expect this to be easy, just because the art style is cute. This game is HARD!
But finishing it is absolutely worth it.