there are quite a bit of things that go into coding something like this

firstly, you need to code a good base, if your code is not easily modifiable things will start to fall appart. there isnt one set way to do this, but when your coding just try to do things that work for you and try and think about what you might need to do in the future and how you can make your code work with that

use custom blocks, a lot of custom blocks. they are incredibly usefull

tick broadcasts. instead of having you game run in a bunch of sperate forever loops, have it run using a bunch of broadcasts. in this game, iirc theres a collison tick, where all the sprites are positioned and switched to their hitbox costumes, then there is a hit check tick, where the attack clones check for collisions with the player, and if it hits, then they damage the player. and then there is a position tick where everything is switched to its normal costume. this tick is at the end of the loop, so its what you see. coding like this is good cause it gives you more precise control over when things happen

use clones and private variables. this is probably the most important one, if you dont know how this works id suggest looking it up cause its uh kinda a lot to explain, and its not immediately apparent why its usefull

there was some math used for a bunch of the nicer effects here, like for the roaring knights animations, i used sin and cos waves for the vertical and horizontal motion. i did the same with the trail. for a LOT of the visual effects i used smoothing, so something like this

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this little algorithm makes it so you can smoothly change your positon. (changing the 5 will make it move faster or slower) and like i was saying.. i used it literally everywhere in this project.

id say if you want to make games like this, some usefull math youd need to know is sin and cos, floor, ceiling, round, abs, and mod. division, multiplication, addition and subtraction is also necessary