The marinade for bulgogi skews sweet, but in addition to sugar, it contains a good amount of soy sauce and garlic. I put together a batch containing soy sauce, garlic cloves, sugar, toasted sesame oil, and pepper; tossed it with the meat; and let it sit for 30 minutes.
I cooked the slices (without wiping off the marinade, per tradition) in a nonstick skillet over moderately high heat, let them sit for a minute to brown slightly, and then stirred until they were no longer pink, just a few minutes longer.
Unfortunately, this beef had a sweetness that screamed teriyaki, not bulgogi. I decreased the sugar from 6 tablespoons to 4, increased the soy sauce from 2 to 3 tablespoons, and for savoriness, doubled the garlic cloves to four and added ¼ cup of chopped onion—an ingredient I had seen in some bulgogi recipes. I pureed the mixture (also common) and then marinated and cooked the beef as before. Much better: The sweetness was still apparent but much more tempered.