After spending the better part of 3 hours trying every different spawn camping/difficulty combination we could think of, this is how me and Squ4rex both got x10 multipliers.
Start off a co-op game in WHITE BELT difficulty on level 1-1. Play through until stage 3/5. There will be a giant rotating red oval with a life bar under it. There will be an infinite spawn on either side of the hallway for colored spheres to roll out of. Both you and your partner take one side and spawn kill the spheres until time runs out. This should get you a x2 or x3 combo each.
Play through the rest of level 1-1 and the next level 1-2 with the gorilla boss, making sure to be extra careful not to get hit.
During level 1-3, play to stage 2/6 quickly, you'll see another red rotating oval and 3 turrets surrounding it. Do not shoot the red oval at all, as long as it is alive it will constantly spawn those 3 turrets around it. One player goes underneath the middle turret and fires upwards, killing the middle and left turrets. The other player sits to his right and fires at the lone turret on the right. The goal here is to get beastie combos on the turrets themselves, NOT the rockets they fire. The turret beastie combo fills up the multiplier bar quite fast, and on white belt you should have enough time to constantly kill the turrets without them firing any rockets to ruin the beastie combo. When one player gets a couple multipliers higher than the other, switch spots while the turrets are busy spawning.
This still may be only enough to get you both to x8 or x9 multiplier before time runs out. If that's the case, finish the level being extremely cautious not to get hit. Level 1-4 is a boss fight, you can only damage him when he's charging up the green sphere to fire at you, and he jumps at you after that, so take care not to be too close. Whoever has the lower multiplier get the kill shot on the boss, it will give you a bit of a bonus to your multiplier.
After that, you can try to make it through 2-1 without getting hit to increase your multiplier more. During stage 5/5, A monster will spawn out of the ground and chase you. If you destroy his 4 eyes, he will start shooting out green blobs. If you and your partner go to opposite sides of the hallway and angle your shots to only kill the blobs, you can keep racking up beastie combos on it and fill up your multiplier gauge more there as well.
If you both don't have x10 multipliers by this point, you can try playing through the rest of world 2, but if you don't have it by the boss in 2-4, you might as well restart. Even on white belt it's fairly difficult to not be hit many times by this bosses attacks.