And the opponent
Is getting no mercy, mark my words
Ain't letting up, relentless

from Eminem (Ft. Lil Wayne) – No Love Lyrics on Rap Genius

Many of ways of saying the same thing:

Em is serious about this rap shit. Anyone who wants a piece of him (rapper or non-rapper) will get demolished.

The “mark my words” could refer to how his words are venomous, so wherever his words hit you they will leave a mark.

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