The important point to getting the All Stages Cleared (Hardest) achievement is this: Get the big weapons and armour first.
*Big Tip 1: Play the Spider cave (the one where you start at the top, and have to work your way down).
At the small tunnel entrance where you start, throw Turrets, Explosives, from the platform you are on, and activate them (also use weapons which can shoot down and do damage, eg. the Assault Rifle with recochet bullets that deflect off wall or the acid gun). Since the Spiders are all below you (at first), just throw them and run back into the tunnel you start in and wait, so you can manage any of the Spiders that foolishly enter that narrow space. Detonate explosives at different times from when they're thrown down, to kill Spiders at different levels below you.
If you have an explosive with a massive bast radious, then this place is PERFECT for it. If you have a co-op partner, have them stay at the other entrance to the narrow cave you're in. They can throw their stuff down to clean up even faster. Once your confident you can venture out, make your way down and you should see some juicy goodies waiting to be picked up. Keep in mind that the level will end if you kill the last spider, so avoid it while you go treaure collecting, then kill it.
*Big Tip 2: Play the level where you only fight Robots (the one where it's evening time, you start next to a broken over-ground highway to your right and a lots of the buildings are leveled). Shoot the Robots with a powerful sniper rifle or a powerful rocket launcher. Try to level the buildings they walk behind and shoot them from as far as you can. This tactic works well and gives you space to avoid their attacks. Collect the stuff they leave behind.
*Big Tip 3: Play the beach level where its evening time,and Robots invade by sea (the one where you start on a hill)
Set up torrets near the edge of the hill to deal with the horde. Wait for them to come close then activate the torrets. Throw any powerful explosives, then fall back as far as you can, and they shoudn't be able to shoot you, but your turrets will be ripping them up. Try not to kill the last one and jump down the hill to collect the stuff.
*Final Tip: Pay attention to where the enemies come from on each of the outdoor levels. They usually come in hordes in a set pattern. Throw your most powerful Explosive in advance, in the path of these hordes, then shoot the enemies with long ranged weapons, until the horde gets close enough for you to detonate.
*Remember that higher difficulties give better stuff so try these levels on hard,hardest or inferno.
Use these places to get some SERIOUSLY POWERFUL new weapon finds. Then you can use these strong weapons, along with your co-op patner (WHO YOU NEED) to do the rest of the Hardest levels.
Good Luck and have patience...your gonna need both!