The octagonal key, along with another oddly shaped key, are carried by high-end grudge monsters -- as I recall, my namesake has one of them. If you can kill him then you more than deserve the contents of the chest. However it's much easier to simply blast the chest with a wand of striking, or using a SoCM to change the box into something softer then kicking (or eating) it open
Really, afaik it takes just about 18 LStr and ordinary unenchanted iron boots of kicking to break the lock open. Or some 20+ AStr to throw the chest on one of the meteoric steel walls for the same effect.
If You can get polycontrol and polymorphitis, for example, from a staff of wondrous smells or a magical mushroom (careful!), I think a veteran or even rookie guard is enough to do it, so don't waste charges on wands of striking, or even worse, socms and wishes. Really! You can get 20 AStr just by blocking a carnivorous plant's attacks with something relatively heavy.
But the chest-polymorphing idea is sum good shit, yo.
Heh heh. This was about to become my next question - how to train those damn stats. I recently took a dislike to metal arms because they're clumsy and artificial limbs in general because they have to be repaired, although ommel hair works rather decently at least. However, with natural limbs, assuming they don't get cut off in a matter of seconds, there are other problems: training them...
Something relatively heavy means what? A steel shield, an iron plate mail, a carnivorous plant corpse? Leg strength can be trained by carrying loads apparently, but it doesn't seem to work for me (quite the contrary, my limbs seem to get weaker and slower for no reason). Agility by running/flying around (then when I stop, it goes down, down, down - and running for free in the world map makes me hungry quick, which results in starvation because bananas are junk and pineapples cost a fortune). Dexterity - how? Endurance - just by healing or school food? Perception - how? (Carrots don't grow on trees and I'm not sure if that helmet of piercing perception +3 really needs any more scrolls into it).
As for sci-talking, there is just a small problem, I don't know how to do it efficiently (i.e. no starvation, and my legs don't degrade back to single-digits Agility before I get a single point). I tried reading the wiki, but my dog is dead (discarded him while flying over the water - yay for levitation), mistresses seem to give no points at all, banana growers barely grow my Int and Cha and then I starve. Well, not always, but it just takes so long.
Are low stats the reason why I have so much problems proceeding? I'm struggling to find a Cruentus altar now 'cause I am going through healing pots so fast. I have a Vermis and an ommel tooth mace +1 in my hands, and until recently I couldn't kill a werewolf in wolf form. A lucky wish from an oil lamp (the 2nd) was used for a broken armor of great health, repaired, hardened to ommel hair and enchanted to +5 with my loads and loads of enchant armor scrolls. I also got a steel arm on the way along (should I change this into ommel hair too? I have a SoCM for this, though I have to change into troll hide and then harden it to do so because my Int is too low). So now I got strong enough to kill the Enner but... last time I got so far, two angels of Loricatus cleaned house for me, but when the level 7 dark frog mage of lots of pain in the ass and disenchanting and teleporting +10 found me, my angels were suddenly gone, I fled up to lv6 where a zombie of Guugzamesh or something executed me. It turned out that without those angels I was just crap. How good do I need to be to go further? What stats do I need to train, and do I need any specific equipment?