This is my best score so far on Mission 6 "The Cauldron" with 9.614.413 in hardcore mode with Armand maxed out. You can see what upgrades I use in the begining. I edited out the cutscenes to save some time.
The video also demonstrate how to get two achievements, Mega Kill Combo (combo x50) and Lord Master of Great Skill (Alien skill level). Basically do a 50x combo in the first half of the level, just don't waste any time for powerups and move fast to keep the multiplier, and don't die once and you'll get the two achievements.
The second half of the level is pretty bad played, I lost the combo a fiew times, so it's definitely possible to do a lot more points on this level.
Thanks for watching.