Korvash is over powered and should be banned in competitive mode if chosen by opponent.
It is generally easier to win by completing the challenges instead of killing enemy champions.
A mix of champions is recommended: at least 1 (but ideally 2 or 3) champions who have abilities to move the enemy to stop them achieving the targets e.g. push, pull or force.
1 champion should be capable of dealing high damage (Korvash, Tentaclot, Tzu etc.) To ensure you can kill champions when necessary.
Movement and good defence is also key, ensure at least 1 champion has a defence card which boosts defence and not just a retaliation card e.g. Kilgore has a defence boosting card but Tzu does not.