Slade would probably trick Adam into hurting himself, or convince him to revert back into his normal form or something. Plot goes a long way with Slade.
In a straight up Battle, Slade wouldn't win, no advantages to take unless Slade had someone BA cared about out of reach or something.
But ya see! Deathstrokes like, so cool and good with prep, he'd obviously make a sound machine that when directed at someone will make whatever they say turn to whatever word is programmed on the machine, in this case, the machine would transcribe Black Adam's 4th wall text into "SHAZAM!" And like, Deathstroke Would totally win. He would do it too, because He's Deathstroke, the coolest most awesome assassin in the world and he is just so cool and impossible to beat!