This guide is for completing the 2 quests without getting any failed quest markers. Though it does negate getting the dwarves to help with the arena, it disappears from the list, so not counted as a failure.
1. Get the certificate from Zdenek
Zdenek gets killed near the arena after you start the Get Junior quest. Loot the certificate from his body
2. Use the certificate to enter the arena
Talk to the guards at the entrance, and tell them you are Zdenek.
3. Talk to Igor to start the fights
Talk to Igor, and don’t make him angry, so convince him to let you fight
4. Fight through all the enemies
Fight the enemies. You can spare the life of the first guy, and he will help you with all other opponents.
5. Loot everything
Grab all of the loot, use the torch/lever on the wall to open the secret stash, loot it, loot Igor’s body, use his key to loot his chest, then leave.
6. Talk to Cleaver, ask for help.
Talk to cleaver, and ask for his assistance.
7. Raid the casino with Cleaver’s Dwarves
KIll everyone in the casino, and make your way to the top room.
8. Release the dwarf and tell him to run
Talk to the dwarf, and then release him, and tell him to run.
9. Talk to Cleaver
When you go back to cleaver, he will give you half of the reward, and Gangs of Novigrad will complete with no failed quest objectives.
He’s annoyed that you didn’t get the dwarves to help with the arena, but oh well.