The Bush Spider is located in the bottom left of the map. You will know you are in the right area when you start to see grass on the ground and scropions/mobs everywhere. After breaking the crystal container, you must fight and defeat the Spider to earn the STEALTH achievement.
Luckily, you can turn on all the assists in setting which will not interfere with your achievement unlocking so definitely check them all!
Strategy*** Spoiler - click to reveal ***The best tactic, along with the other bosses, is to begin pre-firing in the direction it spawns in (in this case, it usually spawns below the platform where you break the crystal, so I suggest standing on the platform while breaking the crystal. The Spider is basically camouflaged to blend in with the bushes/grass so always keep your eye on it. It just crawls around from bush to bush so firing at it is pretty easy. Stay on the platforms to stay safe. Do not let any part of it touch you. After several hits, you'll hear the audio change and this is where the Spider will get more difficult.
AGGRO - When the Spider goes aggro, you'll notice it will start to be more aggressive, going faster to try to get you and it starts to spit venom which arches in your direction. I want to say that it also spawns scorpions, but I don't know if that's accurate, so be careful of your surroundings. Like other bosses, there will be flashes of it disappear/reappearing so just pre-fire in all directions if you lose track of it and you'll hear a sound when you hit your target.
After awhile of evading and shooting, you'll take down the Spider (signified by it stopping and then bursting into white energy). The achievement will pop after defeating it.
Extra Tips - Order of Bosses to fight
1. Kraken (Bottom right)
2. Lizards (Top right)
3. Worm (Bottom middle)
4. Skullbat (Top left)
5. Bush Spider (Bottom left) - Easiest to fight for it's ultimate form.