Hi, first time posting here. Started off on Nutaku, been playing for about four months on DMM server. Just want to say that a majority of players on DMM build multi-elemental teams for raids and that you don't need light to be viable support. This I surmise from checking the team composition during raids, 60-80% use a soul of the opposing element. The thing with DMM is that they have implemented a lot of measures to allow newer players to catch up. This includes a never-ending supply of elixirs and guaranteed R weapons from normal Gacha pulls. I have been able to clear guild order lvl 4 in almost all the elements after less than three months of playing but it will be some time till I can do a full clear since the final enemy hits really hard.
Personally I find it hard to focus on one element as getting good weapons and hime in one element seems to require whaling. If you have that many elixirs and mats you might as well spend them on the best available weapons to beat the current task since it might be a long way off before getting a weapon in your favorite element that is worth grinding for. Also, they seem to be making changes here and there to the weapon stats and I'm not sure that the weapon buffs I've invested in are proccing. (Increased rate for multiple attack with 6 swords, including a phantom sword.) Please don't take my word for it though. Someone more mathematically inclined than me might say otherwise.
If you can read Google Translate Japan wiki, you could probably play this game on DMM. I don't speak Japanese myself so I just guess at what the menus say based on my limited knowledge of kana and kanji. I haven't signed up for an account here because it is troublesome but I will use the same username if I need to reply to this. My intention is not to get people to jump over to the DMM server, just sharing my personal experience as a relative noob to put an end to the speculation about whether it is better to focus on all elements or just one element. At this point in the game, I don't really know. Perhaps clearing higher level individual content requires focusing on one element, but this is the strategy that has worked for me so far. The drop rates on DMM aren't any more fantastic than on Nutaku, just in case anyone was wondering.
Peace.