Oil would guarantee 5 points of damage each round for two rounds. It would take two actions to pull off (one to douse yourself, one to set yourself on fire using a tinderbox), though I suppose you could pre-soak yourself in oil before combat (the effect lasts for up to one minute).
An allied spellcaster could cast Tasha's Caustic Brew and target you and an enemy in a line, guaranteeing 2d4 acid damage per round for up to a minute or until concentration drops.
Or you could just say "Hey DM, I think this encounter is too easy."