Also, on a side note, how do you make such fluid animations? is it just a ton of frames going fast or is there another way?
Indeed there is another way! Most of these enemies are broken into several sprites, which either move around, stretch, or rotate.
To make them move, it's usually a sinusoidal pattern (meaning it'll go back and forth in a wave pattern) like x = sin(timer) * amplitude.
To make them stretch, you can use the image_xscale and image_yscale attributes.
To make them rotate, you can use image_angle, which also usually moves sinusoidally.