I routinely 2-box in LDoN, but only with friends. I'd love to play just one, but often we don't have the healing power so I 2-box the cleric. In my guild are a couple other 2-boxers and virtually any two of us (both 2-boxing: thus 4 toons) can win normal difficulty missions consistently. Right when our avg. group level hit 58 we got bumped into a higher tier and we did have some trouble, but that doesn't last long since the exp is so much better at that point that the lower level people shoot up through levels and you wind up spending lots of time in the middle/upper part of the range getting relatively low exp.
There are two types of "high score lists". I can't believe we have high score lists in a MMORPG, but oh well. The leftmost one is based on your raw number of victories. Given a tie, the person with the lesser number of losses is listed first. If both wins and losses tie, they are listed in some order (maybe alphabetical? Or the more recent person first?) with the same number. The righthand ranking is from win percentage. On my server, the top 100 is dominated by people who win without loosing - ever, thus they all have at or near 100% win percentage. How do I know this? One of my guildies was all excited about this and we did cartloads of missions in one zone until we all achieved over 1490 points there. Took a while but he is high up on the win % list for that zone. I personally wish they would toss the whole "high score" thing because I would like to include people. If we start a mission with 4 people, if a guildie comes online say 10 min into it, I'd love to be able to quit to pick him/her up. But sadly not if you are trying to preserve your perfect 73-0 record.
Any mechanism to do this would rock. Say: they join in the first 30 min, but only get 2/3 the points? You issue a special command as group leader to invite them to the dungeon and they accept or decline it (obviously you have to have an open spot in your group). Or you can quit the mission within the first 30 min and not take an official loss - you name it. Or of course they could dump the high score list.
As for mission difficulty, you should try normal missions first but I would encourage you to do "hard" missions if you have the group for it. (Your group leader selects "hard" from the difficulty pull down tab when requesting a mission). I don't have first hand pre-level 50 experience with LDoN, but I have heard the missions are pretty easy to win but you get jack for points. Ergo, your best option may be to just focus on leveling up. By taking a hard mission you may more then double the ammount of exp per kill you get - so maybe you loose the mission, but get more exp then you would normally would and thus level quicker to a point where you do get pretty good points per mission.
Beware of the level tiers. Each tier is 5 levels wide and dictates the level of the mobs you fight. You can jump up to the next tier by selecting a hard mission. We seem to get about 50% more points for doing the next hardest tier. Basically, if you average group level is from, say 53-58, you get predominantly level 50-51 mobs (I think it is around the mid level minus 5 but I may be off here). Maybe your avg. group level is 53.1 - this mission may be fairly tough for you, but if your avg. group level is 57.9, it may be trivial.
Further, a single person dinging or a small change to your group can bump you into the next tier. If you've done a great mission and, say, someone dings and your group level hits 58 (avg) but then you go back all excited and select a "hard" mission, you just jumped up 2 tiers! You're fighting level 58 mobs in stead of 48 mobs from last mission and probably not too happy. At that level, you may start having trouble landing any stuns to interrupt gating/complete healing mobs, any casters of low level will have serious trouble landing many spells, etc.
I kind of wish they would put the level of your mission there in place of the "normal" and "hard". When we're at the upper end of a tier, we usually want to try hard missions because of the increased exp, but if anyone dings we don't know if we are already at the next tier or not. I tried making a little table here with level and tier but it doesn't seem that anyone knows for sure.
Some of my numbers here may be off. If anyone can fix them please do and I'll edit the post.