Why hit with 1 shot? I prefer 2 shots, as you can time it so that the mobs drop their gold at your feet - thats what I call showing proper respect.
As to lvling spots I like the forests round IT, 2 shot pink/red mobs with no problem, not aggro, just stand in the middle of a field and watch them cover your feet in gold!
Lobomol is correct, 2 shots are better than one. You can use your standard nuke on the first shot to get the mob to a low HP level then your Overhit nuke for the second shot, thus increasing the exp/mob by up to 15%. That is the same as levelling at 85% instead of 100%.
Overhit exp bonus is 25% iirc and I personally wouldn't recommend it. Or just die a **** lot more than I did, because I'm having problems with SP :) I don't quite get your point why two shotting would be better than oneshotting though. You spend more mana and more bsps obviously and have a higher risk of having several mobs hitting you at the same time. It isn't easy to find mobs that are one hit kills though, but that depends on your gear and buffs too. As a lvl 40 SH you can still grind in Field of Silence, onehitting the ogres and lizardman snipers, they should be 'ok' until lvl 43 or so.