In game maker, in an IWBTG engine, it's a good place to make a boss.
If you experience any problems, you just ask me.
Here's a quick guide.
Create event is when it's just created, the first step that it exists. A step is a little piece of a second, room_speed determines how many steps there are in a second, usually 30 or 60.
Step event is the same code that repeats every step.
An alarm is something that happens after a certain time, you can determine what alarm goes of when with code.
And a timeline is litirally a time line, with as data the amount of steps from start of timeline to end, and with it each step the information on what to do.
In create you set the lives to a value, and you let alarm 0 count off starting from 50 or something
In step you set that it gets damage when touching a bullet, and it makes the bullet dissepear, and if the boss has 0 lives, it dies, or it explodes or something
In alarm, you start a timeline (all the attacks), if your timeline is 120 steps and an attack each 60 steps (two attacks), and you want to do the next timeline 50 steps after the first timeline, you set the second alarm to 120 (timeline) + 50 (waiting time), so 170 waiting time to the next attack
In timeline, you set all the attacks, for example bullets flying at the player
That's the basics, but the way how you code that is with commands, for example, if the lives are 40, then you write in a code in create lives=40, you can look others up or just ask.
It's a basic explaination, I hope I helped.