The sentence "I have mastered" is correct and usable in written English. You can use it when you want to express that you have achieved a certain level of skill or knowledge in something. For example, "I have mastered the art of making delicious pies."
I have mastered the Cruyff turn.
But I have mastered my craft.
I have mastered the base-10 numeral system.
If "wobbly warrior" is a yoga pose, then I have mastered it.
I wake up earlier than the others, usually around 5 A.M., to perform the one household duty I have mastered.
I don't want to say I have mastered the craft [of writing books], but I feel a lot more confident.
"It's something that I've mastered," Ms. Keaton said.
It's not a genre I've mastered, the eulogy.
Every board, every crack I had mastered and danced on.
And once I had mastered it, I began actually to look forward to the laundry.
I'm allowed to gut the whiting, and when I've mastered that, a few rays.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK