There is no need to use a breaker bar or any kind of extension to get your stem tighter. You are quite likely to damage the stem if you use anything other than a torque wrench, a standard Allen key, or a multi-tool.
Remove the faceplate from the stem, centre the bars, and replace the faceplate. Tighten each bolt a few turns, following the pattern on the right. Continue tightening until you reach 75 inch/pounds (8.5nm). This is approximately as tight as you can get the bars using a multi-tool.
Maintaining equal load on all 4 bolts is critical for durability. Please ensure that the gap between the faceplate and the stem body is equal as you tighten the stem down.
Over tightening the bolts will lead to damage.
Never ride a damaged stem.