Sett excels in early to mid-game trades with his potent passive and devastating W (Haymaker) damage; a full-grit Haymaker can be a game-changer in teamfights.
As an immobile champion, he's vulnerable to kiting and poke; engage him when his W is on cooldown to exploit his weakness.
Effective counters include Mordekaiser, Volibear, and Jax, who can negate his W or maintain distance; always be wary of his Flash-Ultimate engage.