Is there like a Merlin guide or how to use Merlin effectively?

Typically speaking, outside of farming, your ideal situation is always one DPS servant + two servants buffing him (maybe three, with a plugsuit). You can't always do this, of course (nodes with multiple classes, for example), but since Hero Creation is a single-target skill, you will get the most mileage out of Merlin when you can be relying on a single damage-dealer.

He's considered game-breaking because (a) a Buster/crit-focused servant can do NP levels of damage with normal cards, and (b) especially with a DPS/Merlin/support Merlin setup, he completely breaks the balance between servants who have Buster NPs and those who don't.

It's easy to use him, press his 1st skill + his 3rd skill on your Buster Carry and win.

To put it more simply :
- skill 1 + 3 = win.

Sorry, jokes aside 🤣 and you can try some Merlin tricks below ( the Memes is the truth, though ).

- Nope, it depend on the servant and servant like Gilgamesh and Raikou ( AOE Premiere SSR Buster servant ) really appreciated Merlin supporting skill.
- as long as it is Buster servants, no matter it is AOE, ST or Non-damaging NP Buster servant Merlin can turn them into monster ( i've done some silly thing with a Merlin + Leonidas as my DPS, Leonidas hits like a truck ). Mind you, Leonidas is a Tank dude.
- Waver + Merlin is also broken as fuck !
- you need to use him pretty often, so you will know his playstyle well and utilising him more properly. Practice makes perfect !
- know your Merlin first, then know your Mystic Code, then know your Buster Carry and team member, then know your enemies.
- he is a walking meme, his name is Memerlin and he is the strongest servant in FGO be sure to take care of him well.
- stage mechanics is also a deciding factor, like a breakbars and more.

Don't worry, it's pretty easy to use him and you also can almost bring him everywhere.
Take your time and enjoy his support !

He is game breaking as he is able to perform many roles that would typically be performed by several servants, and do them to a greater degree. Being able to charge NP with his 1st skill when needed is invaluable. Having a teamwide invincibility on demand with 2nd skill is very useful when enemies are about to use NP. And his 3rd skill turns any buster dps into a monster.
The powerful thing about his NP is not just the effects, but that the effects can last for 5 turns and stacks with multiple uses. Which means stacking healing, passive star regen and np regen, and this is done by only 1 servant.

He's meant to be a buster crit support, but I use him in arts stall and he's still op. This isn't game breaking enough for you?