Thanos:
Has the infinity gauntlet, which gives him total control over all of reality.
Immortality
Invulnerability to even adamantium, the strongest substance on earth.
Super strength.
Darkseid:
Omega beams: just insanely powerful and injure you to the brink of death, and don't stop until they hit their target.
Immortality.
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Infinity Gauntlet. You might as well set Deathstroke against a bunch of teens that forgot to bring him lunch money.