12-16k SP per wave taking 4 to 5 minutes, easily 140k per hour, potentially 180k+
The solo method (repeating wave 4) posted elsewhere is a great method to earn SP, however I found that I didn't get on with the suggested map. So I switched over to 'Dead Rock', discovering a way to increase your chances of getting six kills with one drill from the Penetrator most of the time. The Penetrator kills add a 100SP multiplier for each enemy killed, so hitting all six in one go, gives you 3600SP (sometimes more with other bonuses). For myself this now means an average of around 13.5k SP for the wave and a highest so far of 16,500 SP, though I suspect you could get another few k on top of that.
The key to doing this is to get all the chasing enemies to form a nice orderly line ;)
This can be achieved on 'Dead Rock' by funneling them back and forth through the gap between the 3 stacked cases and the nearby cactus (the one that is next to the pipe that crosses the pathway, which you can slide under). This gap forces the enemies into single file most of the time, at least enough of a line to take them all out.
So to begin play Anarchy mode, private match on 'Dead Rock' and complete the first 3 waves however you like. Upgrade only speed, NEVER defense as it takes ages to be killed at the end of the wave, purchase and upgrade the Penetrator gun, but don't bother with its special attack. Once at wave 4 you want to follow the description below in order to kill 6 enemies with one drill shot and gain lots of SP. When you get to the last enemy (the one with a team challenge) just let them kill you. This way you will keep all the SP earned, but restart wave 4. Rinse and repeat.
Start off by running around the area next to the 3 cases (its near the man-eating plant) and group together the 6 enemies. Now start 'sliding' through the gap between the cases and the cactus, around the vortex, between the vertical dinosaur bones and a vortex crane, up towards the metal steps leading to the main platform. Then turn around and wait for the enemies to catch you up. None of them singluarly do enough damage to kill you, just be careful not to get surrounded. Wait for the enemies to get passed the aforementioned vertical dinosaur bones and vortex crane, then slide back towards the gap between the cases and pipe.
Don't stop sliding till you are some way past the cases, as this will give you enough time to turn and line up your shot. Make sure you avoid sliding into enemies!
You can often wait for one enemy to almost reach you to ensure the rest are lined up behind them (and sometimes get toxic love bonus, plus Root Canal), though if you have any doubts its usually best not to fire and instead repeat the process to try again.
Obviously you can take a shoot after sliding in the opposite direction and should be easy to get 4-5 kills in one go, but its not quite as reliable. If you ever find you've only killed 3-4 then get the last two/three to line up in front of the vortex
and shoot them with a single drill, one will go into the vortex and gain +150 SP over just drilling. I do this by hanging around on the metal grid next to the vortex opposite the dinosaur head. Wait for the enemies to swarm around, then slide towards the dinosaur head, turn and line them up and shoot.
A few notes..
Ideally you do this on wave 4 with the green and flail guys, sometimes you might get enemies using machine guns and these can be a right pain, mainly as they don't follow you like the others do. If you have machine gun guys you can continue or restart Anarchy mode and battle through waves 1-3 again and hope you get the flail guys instead. Which enemy you get appears to be random.
The flail guys will explode when hit with the drill, so you must NEVER shoot the line if the flail guy is at the front and
should avoid doing so really until they are near the back of the line. All this takes is a little bit of patience and running back and forth.
All the enemies on this wave are pretty harmless and can easily be dodged or dummied. You can survive many consecutive hits from them, so don't panic and take your time.
Upgrade your speed as this can give you a good edge over the enemies when running/sliding.
I found the auto-lock feature more annoying than helpful, so when aiming, point at the ground first to avoid locking onto an enemy and then aim manually.
Enemies don't have to be in a perfect line as after the drill passes through one it can veer at a slight angle into the next.
Just ensure that the enemies aren't grouped together in a pack, each one needs to be in front of another. In fact its possible (though needs luck) to shoot a drill around the vortex area in a circle ;).
Level 65 requires 1.9 million SP.