Thief Silver is the challenge in question. Good luck; this is one of the trickier Silvers I think.
The Level 2 guards are marked on your map. Use WEYTWUT a bunch of times to get to them. One of the tricks of this level is placing the monsters you move so they're out of the way. You might have to use a potion to kill the first one; make sure to slow him so killing one brings you to Level 2. After that, you're just trying to kill monsters a level or two up to bring yourself up to 7 so you can kill the boss.
In addition to the LEMMISI chest you start next two, there's a L1 chest with GETINDARE, which is helpful in a clear way. The L3 chest contains IMAWAL, and the L5 chest contains ENDISWAL. ENDISWAL has three purposes. First, it helps you access additional blackspace. Second, it helps you explore behind monsters and chests. Third, with three stacks and a GETINDARE,, you can often kill a monster two levels above you. Save IMAWAL for cases like this, where you can brick a L2 (-2 XP) to get 1.5x XP from, say, a L8 when you are L6 (8+4 = 14 = +7 bonus XP)
The L7 chest contains a Pendant of Health, which helps your potions go further, and the L9 chest contains BURNDAYRAZ, which you need for the boss. If you have almost all of your potions, you can kill him in a straight-up fight at L7, although having a mid-fight level up will make it a lot easier.