I was able to get 1.2mil on my first attempt using these methods and I only killed about 6 of the bosses, meaning I could've gotten an even higher score, but thankfully I don't have to touch this absolutely abysmal content next year.

Also, prioritize Epoch > Broggok > Harvester adds. Epoch debuffs the healer which can kill Garrick, Broggok drops big damage pools, and Harvester adds chunk HP with their AoE when they're both up and he goes into Overdrive, which can also kill Garrick.

Unfortunately you do have to babysit Garrick because in their infinite wisdom Blizz didn't just let you control her pathing with the pings system.

- Use horns of plenty if you notice that the last standing boss is about to die and there's no cogs on the ground to summon another. The first time all the bosses die, you get a "bonus round" which summons two more. Any time after that, it ends the match.

- Use the knockback from the indisgestion debuff to reposition Garrick or the bosses as needed, and be mindful not to unstuck the cheery bombs.
- Keep your time as high as possible using the stopwatches and the cogs. Only grab cogs if you're down a bar of time.

- Talk to the Troll outside and make sure to take the "indigestion" challenge, eating the hotdog. This increases the score multiplier. Combine with the can of amps for movespeed.

- Use the mimiron head things to keep forcing the bosses into the stacked circles on cooldown
- DO take any regular cogs on the ground when you see them
- DO NOT take any SPARKING cogs on the ground, as those summon the zapping Goblins that path to the bonus zones and chunk everyone.

Not sure if it's much help, but I managed to beat the score by doing the following:

- Prioritize getting the two cheery bombs to overlap with one another before anything else.
- Once they're in position, use a master key to move the traveling bonus zone on top of the two stacked cheery bombs.