People must be getting tired of me shouting this up here, but it is very beneficial for all FS out there to start investing their time and effort in their mercenaries as they are very, very useful in leveling!!!
1. Even without double EXP event, you still gain more EXP soloing in single dungeons.
Single dungeon - Stage 7-1 to 8-10 give you varying EXP gains from 3-5exp per 30min (w/ Hero book) starting form base level 82up
Dullahan Grind - A gain of 2-3 exp w/ Hero book (even lower if you're not a running FS and your party only got one wiz)
Single dungeon - unlimited. You're not penalized with 2% off your exp when you die. Just stack those tokens to revive yourself
Dullahan Grind - its a heartbreak just losing that 2% because the wall fucked up. Or the server was laggy and you're running only to find out you're already dead.
Single dungeon - there's a chance to get rare items from gold chest. Even eluniums and oridecons.
Dullahan Grind - err well, you could get loots but there's really not much in dulla map to hope for. R3s?
Single dungeon - okay so you now solo and you just have an AI for company. So what? Soloing in dungeons gives you more leeway to chat with your guildies, to have a private word with your friends etc. It's not like you've suddenly gone AWOL from socializing with everyone.
Dullahan Grind - chatting while running? not a good idea >< Besides all your party mates are probably AFK so basically you're still alone. To hell with them! gg
Single dungeon - no dramas! unless you treat your mercenaries as pets and cry every time they die. lol
Dullahan Grind - oh there's just lots of causes of drama in here. Wall suddenly dc and you are all shouting. You got dc and they're all shouting at you when you get back. They say it's fine when they die but it's really not. Same case for you too lol.
Single dungeon - depends on the keys you got. It's quite limited but it's enough to enjoy leveling in a way that won't saturate you. You could rest up while waiting for your keys to fill up (unless its double exp event and that's when you feel the urge to buy diamonds, but it's so worth it hehe)
Dullahan Grind - you could always do this but it's very hard to find a good party to start with. Sometimes it takes a really long time to assemble a party unless you got a nice permanent party or a very active guild.
Well these just cover some of the pros why we should switch to dungeon leveling. I'll update this to add more argument to this cause.