In the absence of documentation online, I decided to write my own little guide on how to slide reliably and get the Grassy Knees achievement.
If the delay between letting go of the direction key and holding down is too large, you just crouch in place. If you start holding down before letting go of the direction key, you start crouch-walking (which does not count as sliding). The timing is not difficult, but takes a little to get used to.
The achievement ‘simply’ asks you to get 650 kills while sliding. This basically never happens unintentionally (I only got 31 in 39 hours of playtime, before going for the achievement), since a slide does not last long and you need to get the killing blow during it I’m pretty sure. While you can start trying to do it anywhere as you can just slide on even ground, you can make it easier by sliding down a slope to make the slide last a little bit longer.
The ‘Tropic’ map is suited well for trying to get sliding kills, as at the top there are two building with sloped roofs, one at each side of the map. So, after every death I would run to the top of the map and start to run over the rooftops waiting for enemies to come near. Then just continue until you are killed and repeat.
It can sometimes take ten or twenty seconds before an enemy comes near, so to increase the frequency I set up a custom deathmatch game, solo vs. nine enemies, with a goal of 100 kills. To aid survivability, I recommend increasing spare ammo and health regen. I set the respawn timer to 0 to stay in the flow easier and to get more enemy encounters.
In terms of weapon layout, I would highly recommend a build with some form of one-shotting weapon. Personally I like the high-damage revolvers such as the .500 Torro because of its high accuracy, damage, and fire rate amongst such weapons. I tried the boomstick shotgun as well, but half of the enemies dont like to come really close. In any case, being level thirty with a (near to) level 30 weapon and level 30 hollow point accessory really helps with the damage.
I don’t think any one character is has an advantage because of their abilities, so that’s really just personal preference in the end.
Now, just start grinding kills. It still takes a long time, but at least you know what to do 🙂