I saw a video on the show and deathstroke was fighting on even grounds with Green Arrow and some impersonator. I guess it could be because the show is about green arrow but could someone tell me. Is he underpowered in the show
Well it's like Nolan's Batman in that the show is primarily grounded in reality - they've already said no superpowers exist in this world, at least for now.
I was thinking this as well.
And to add my own thought, I dont think Slade is Deathstroke yet. Its entirely possible in season two he's been augmented or physically elevated to a level more recognizable, but even then I highly doubt we'll ever see Comicbook version Stroke in terms of feats and abilities.