Go to Chief Paroxysmus' Lair North of Papua, you need a rope to get in or you can buy one from the guy at the door for 5k, then follow it down to this point. It's usually clear of monsters but if not lead them away.
Go into Cat/Dog Form for the HP regen and stand roughly where I am in the picture. Here gas will burst every so often and kill any images you summon. Some tiles have more gas ticks than others so find one that only has one, if you need to you can simply step one tile out of it to heal up but the regen should take care of it. Depending on your HP you may want to wear only items with no durability as it may wear them out unnecessarily.
If you have Hiding/Stealth on the template as well you may be able to do it in that too (can't remember).
If you are on Atlantic there is/was another location known as Yriggor's dungeon which also had gas tiles, but I can't remember where it is.
If you are training this on a Stealther and you want to use Shadow Jump for training, aside from throwing on lots of LMC/MR and 4/6 casting you can make a UOA macro to Shadow Jump Target Relative Location (where the relative location is 1 tile N), then 1 tile E, 1 tile S, 1 tile W, with a delay of about a second (1000) between each. You can then repeat this macro inside the walls of Luna (the inner walls near the bank) and it will work in such a way that even if you fail the jump and it goes to jump on to a tile that is blocked, the next line will always be moving away from the obstacle.