The Baconing

Not hidden, impossible to miss but you have to use them
You will find the last one after defeating the final boss

Storyline

You can't miss it
Use 3 punch cards on the Replicator, in this order : 3 > 5 > 4

Game Over Man!

Just beat the game, unlocks Insane difficulty.

Death Blossom

Play through again on Insane to unlock this achievement. You must start a new game on Insane and not change the difficulty.
Just only do the main quests, and pick Health and Damage upgrades

Optional Quest

Finish all of the "Twelfth Labor" Zeus' quest (there are 3), in Valhalla Heights.

Optional quest

North of the Forbidden City.
You can complete the first three early in the game but I'd recommend waiting until 20 to finish the rest of them (use the Arena outhouse to get there quickly).
There are 20 rounds total. Achievement unlocks when you open the chest.
Round 20 tip: Morbo takes heavy damage from the nature barrel.