you don't need to get 1,000,000 points in a single level, you just have to obtain 1,000,000 as a current cumulative score across all levels. for example, with highscores like this,

• The Workmen Ward - 100,000
  
• The Melting Pot - 200,000
  
• Cinderbrick Fort - 200,000
  
• Kid's Who Knows Where, Kid - 500,000

you will get that achievement

pause -> leaderboards -> Score Attack Mode
like this:

• foe's power (a little Gasfella gives less points than an Ura warrior)
  
• activated idols (the more you have the longer combo meter will stay)
  
• combo meter (killing enemies increases the combo meter)
  
• received damage (every piece of damage decrease the combo meter)

so in theory you should activate as many idols as you can, take as little damage as possible, kill enemies as fast as you can (to keep combo meter up), and, ideally, finish the level with a single combo meter without breaking it (tho it might be very difficult or plain impossible on some levels)

I believe 7,000,000 is theoretical maximum you can get, but you must never lose your combo meter, take no damage, be very fast and know every single enemy spawn in every level

score of 5,000,000 - 6,000,000 is more possible (my high-score is 5,866,697), if you don't want to be super grindy

...but remember you only need 1,000,000 for that achievement

here is a rounddown of my high-score for every level:

• Wharf District - 58,330
  
• The Workmen Ward - 125,127
  
• The Melting Pot - 324,133
  
• The Sundown Path - 118,961
  
• The Hanging Gardens - 5,297
  
• Pyth Orchard - 101,200
  
• Cinderbrick Fort - 192,105
  
• Langston River - 103,353
  
• Prosper Bluff - 9,365
  
• The Wild Outskirts - 41,863
  
• Jawson's Bog - 143,566
  
• Roathus Lagoon - 35,909
  
• Point Lemaign - 42,355
  
• Colford Cauldron - 398,262
  
• Mount Zand - 154,252
  
• Burstone Quarry - 45,412
  
• Urzendra Gate - 22,991
  
• Zulten's Hollow - 322,059
  
• The Tazal Terminals - 509,016
  
• Kid's Who Knows Where - 1,079,025
  
• Zia's Who Knows Where - 585,964
  
• Zulf's Who Knows Where - 569,389
  
• Rucks's Who Knows Where - 853,972

Kid's Who Knows Where grants you, with a right strategy, about 1,000,000 points (and you can finish the Hard Bargain achievement with no further hustle). check that guide for the details
https://steamproxy.net/sharedfiles/filedetails/?id=526458977

• The Melting Pot is the easiest place to get 200,000-300,000, most probably the less grindy area possible
  
• Pyth Orchard is another good spot for no-sweat 100,000 points
  
• Colford Cauldron is a great level where you can get your combo straight from the start to finish and obtain nice 300,000 - 400,000 points. the only downside is the level is quite long and you need to remember lots of spawns
  
• the very last level - The Tazal Terminals - is a gold mine in which you can get 500,000 points. it is also overly easy to beat (especially the last part of level with a Battering Ram)

first weapon of choice is Brusher's Pike with the following upgrades (has a damage boost and ignores armor)

the second weapon is either Fire Bellows with Burning Carousel secret skill and with the following upgrades (we don't care about Fire Bellows itself, but we boost up its great AOE secret skill):

or Fang Repeater with Bolt Burst secret skill and with the following upgrades (again, we don't need the weapon itself, but we do need its enemy-seeking AOE skill):

use this:

• Olak is out. no excuses. because of enemies' invulnerability - it messes with a combo meter and makes losing it easier
  
• Acobi its an optional candidate to be out, because of his little red bombs that break your combo meter quite fast. I have played without it

to be fair, anything will do, I believe the following spirits are must:

• Leechade - helps to survive
  
• Bastion Bourbon - compensates Leechade negative effect)
  
• Cham-Pain - easier to kill enemies
  
• Bull Brandy - more defense
  
• Squirt Cider - more health
  
• Whale Ale - makes Burning Carousel & Bolt Burst stronger;

other spirits could be whatever you like, I prefer

• Dreadrum
  
• Werewhiskey
  
• Black Rye
  
• Doomshine

and so, my setup is

if you want to thank me...

...then check my guide about fixing various game issues: launching, crashing, optimization, stuttering, gamepads, etc. the guide is huge, but the topic is broad to begin with
https://steamproxy.net/sharedfiles/filedetails/?id=1935376439