To earn Lightswitch (and Spartan Officer from the retail disc at the same time) you must earn 50 EXP in a single playlist. You can check this by selecting your gamertag in a lobby and looking at your stats, and the area at the bottom right will tell you how many EXP you have in that playlist. Your "Total EXP" does not matter, what you are interested in for this achievement is to the left of that stating e.g. "Rumble Pit EXP".
To do this solo, I would recommend team playlists and praying that your teammates aren't playing on donated consoles at the local mental ward. Each team win is 1 EXP closer to the achievement, though each team loss is simply lost time because you get no experience for it. Free for all modes may be your thing if you can consistently score well (not necessarily first place) because the top four spots all receive 1 EXP in Rumble Pit.
However, if you're interested in a more time efficient method and don't mind a bit of boosting, you can party up in social and play rumble pit. You want to veto anything that is not slayer or swords because every other game type makes it difficult or impossible to distribute the experience points. If you end up stuck with a gametype that you can't balance, just pick one person to own everybody and end the match as fast as possible. Take note, however, that all five DLC packs or the ODST bonus disc are required to play rumble pit.
Assuming everyone cooperates over the length of the session, you can finish this achievement from 0 EXP points in 3 hours for all 8 people involved. In rumble pit, the top 4 all receive one experience point at the end of the match. This is most easily accomplished by everyone getting one kill to tie for second, then having one person go nuts until they win the match. It is very important that everyone ties for second, because the leaderboard is still enumerated. To explain what this means, I will give a small example:
#1 gamertagA 20pts
#2 gamertagB 2
#2 gamertagC 2
#2 gamertagD 2
#2 gamertagE 2
#2 gamertagF 2
#2 gamertagG 2
#8 gamertagH 1
Notice how gamertagH is eighth and not third, which means he gets no EXP for that match.
Ideally you want no guest accounts and no zero scorers to stave off the ban hammer until after you're finished. I know enough of network programming and pattern recognition to tell you that it is not difficult at all for an automated system to tell if a guest account is simply *there* as opposed to someone actively playing it, and rubberbanding your controller does not fool anybody at Bungie. To get this done with maximum speed and efficiency, I highly recommend 8 actual people, and that each one of them remains to continue the method even after they are finished.
Why rumble pit? Free for all modes are the only way you can get everyone an experience point at the same time. Though it is likely that many people have a lot of existing experience in team modes like Team Mythic from attempting the achievement on their own, it makes a poor choice for boosting because the losing team from each match will get no experience. In addition, the teams will not always be distributed the same, and it is very likely that a few people could make out like bandits and a few others could get the shaft. It's extremely unpredictable and likely completely unfair.
To elaborate on Halo bans, all that happens when you are caught boosting is that you are banned from gaining experience for a short period, usually two weeks. You are not banned from XBox, banned from playing Halo online, nor are any of your EXP totals reset or adjusted in any way. As a matter of fact, you can continue to unlock multiplayer achievements, so you could, for example, participate in a session to unlock DLC achievements while EXP banned.