Nobody Saves the World

The Mechanic is unlocked by obtaining six total medals and then going through the first door in the Colosseum. Once you unlock it, you'll need to (fast) travel back to the main world to be able to work on challenges (Form challenges are locked inside the Colosseum). Just like in the base game, you'll have a series of challenges to complete while making use of new attacks and passives, and completing them all is required to reach S-Rank.
 
Thankfully, just like with the base game, many of the challenges can be completed in the northeast corner of the map. Specifically, refer to the image below, which is the same place I recommended using in Super Wiz (100G) for where to go to have a larger group of enemies that you can use for the majority of the Form challenges. Some challenges for this form, unfortunately, are more of a pain and can't be done as easily in this area. I have provided more details below on each challenge.
  

1. Overhaul I: Hit baddies with Overhaul - simply mash in the area mentioned above
2. Overhaul II: Heal your Rattler Turrets with Overhaul - this one is misleading. Hitting an enemy with the fourth hit of your combo is NOT how this works. You need to place a turret and then actually attack your turret to heal it
3. Overhaul III: Hit baddies with Overhaul's fourth hit - use the area above. Mash away from enemies and only move in at the end of your combo so you don't kill them before the fourth hit
4. Salvage I: Trigger Salvage! Salvage can make baddies and Familiars drop an additional health item - no real way to speed this up unfortunately. Just grind it in the area above and hope for the best. Obviously upgrading this passive (assuming you have plenty of spare upgrade tokens) will make this go quicker
5. Rattler Turret I: Construct Rattler Turrets - nothing special here, but the Quick Cooldown passive will make this go much quicker. Although, this will come naturally while working on the below challenges
6. Rattler Turret II: Hit baddies 15 times with one Rattler Turret - the area in the image above works well for this. If you're concerned about the turret being destroyed, either attack the turret with Overhaul to heal it, or use Blood Pact passive while you hit enemies to heal the turret
7. Rattler Turret III: Have 7 Rattler Turrets alive simultaneously - while you could try to keep them alive long enough, the quickest and easiest way is to equip Quick Cooldown and have it at level IV, then also equip Magic Life (uses health when your mana runs out). Now you should be able to spawn a new turret every 3-4 seconds, allowing you to easily spawn 7 before the first one runs out of health
8. Barrier Coil I: Block enemy projectiles with Barrier Coil - the area above won't work since the enemies don't shoot. I fast traveled to the north central (final) area and used the large enemies that shoot balls at you
9. Barrier Coil II: Hit baddies with Barrier Coil - simply use the area above, go up to the fire and drop Barrier Coil there and all the enemies will run into it
10. Barrier Coil III: Block 5 enemy projectiles with one Barrier Coil - once again the area above won't work for this since they don't shoot projectiles. I went inside the Dead Dragon dungeon, and the first room always contained enemies that would shoot sets of 3 projectiles at once. This made it easy to get 5 per usage each time. Let them kill you if/when you run out of mana
11. Custom I: Express Service: Kill baddies with the Fleet Footed passive equipped - simply grind it out in the area above
12. Custom I: Mechanical Advantage: Defeat baddies with Ranged Abilities - simply grind it out in the area above. I used Arrow Flurry but any ranged attack will do
13. Custom II: Familiar Improvement: Heal non-turret Familiars with Overhaul - you don't even need enemies for this one. Just use Hat Trick and spawn Familiars nonstop until you're out of mana, then just attack them with Overhaul to heal them until this challenge completes
14. Custom II: Give 'em Heck!: Hit Slowed, Stunned, or Feared baddies with your Rattler Turrets - easiest way to grind this out in the above area is to use the Slime Slide attack from the slug to slow all the enemies in the area, then drop a turret and watch it ravage the slowed enemies
15. Custom III: Auto Repair: Heal Rattler Turrets with the Necromancer's Blood Pact - easy enough to grind out in the above area. Drop a turret (preferably all the way down so it doesn't kill enemies before you), then just go around hitting enemies to heal it, since its health continuously drains anyway
16. Custom III: B O N K: Hit baddies with Impact damage - you'll first need to equip the Strongman passive, as this gives all attacks Impact damage that knock enemies back. Now either use Tail Swipe, Pump Up, or anything else that knocks enemies back, and grind this out in the area above. I would knock enemies into the three rocks in the center