俺に勝てるのは俺だけだって英語でなんて言うの？

In order to explain that there can be only one person like you, the common phrases are:

I'm one of a kind!
There is no one who can be the same as you.
A kind - a type, a species.

I'm unique!
You are special and an individual.

There's only one like me!
This means that there is only one type of person like you.

I hope that helps!

You can say :

'I'm the best.'
'Nobody can beat me.'

Indomitable =impossible to subdue or defeat.
"She's a woman of indomitable spirit."
Self destruct button = a characteristic in a person that makes them likely to fail because of their own actions:
"At least he reached the semifinal before hitting the self-destruct button."
Achilles' heel = a weakness or vulnerable point.
"The Achilles' heel of the case for nuclear power remains the issue of the disposal of waste."

Simple expressions of self-motivation:
I am the best.
No one else can beat me.
No one else can be me but me.
There is only one me.
I am the greatest.
I can only be me.

You could phrase it many different ways, and by saying, "I am the only one who can beat me", you are saying that you are strong enough to put up with pressure from other people.