If you're a newbie, you may want to focus on upgrading your Main Character's physical strength on levelling up until it hits maybe 18 or 19, and then focus on Magic and Health (with the occasional Agility boost), because otherwise Day 3's boss fight might prove tricky. Also, any Demon you can obtain with either Evil or (I think) Chaos Wave as an ability will be your friend, because you can snipe enemies from two or three panels away, and they can only retaliate if they have a demon with the same ability on their team. As far as passive abilities for your Main Character, Hustle and Tailwind are useful if your MC is lacking in physical strength or agility.
Cracking the Elemental Dances (especially Holy Dance) is also useful, particularly against enemy squads that only contain one member, as they'll take all the hits. I'd also recommend cracking Pierce onto one of your characters, so if there's an enemy squad leader that you CAN kill flanked by two that might be more tricky, you can deal full damage to the squad leader instead of the reduced damage normally dealt.
S'not much, but I hope that helps
I agree partially. Leveling up the MC's STR is a good thing up until about 15 or so (if you can score any attack increasing spells or so, do nothing but have your demons use them in the battle with the boss in question), but there's too many moments in D.Survivor where you have to rescue someone, so Genma or Avian or Wilder demons are a must, so grab the latest ones and fuse'em, you'll need the movement space (I don't know if Avian's abilities and Wilder's abilities stack, so it might be better to just have two characters to have a Wilder at the start and the other to have an avian, and then when you get the 4th member, give'em another Avian). Chaos Wave and Evil Wave are not helpful when you need to cross the whole field to get to someone.
The problem with the Dances is that they tend to only hit twice, not three or four times. Pierce is another good ability, but the major problem with that is by the time you have access to it, most of the enemies will repel attacks instead of null, resist, or absorb, so having an Almighty attack works better. Speaking of which, GET DRAIN ASAP. In other SMT games, Drain isn't that useful of a spell, but here, with the right members, you can drain the entire enemy party of their MP and spent none whatsoever yourself in the end.
As far as ending's go, you can try Noya's (or whatever the cousin's name is) path first, since the beginning chick leads to a bad end and bad NG+ (won't spoil anything, but as far as rewards go, her's sucks since she doesn't do much, while every other path gives you +1 to move). While it doesn't really matter what path you go down, Noya is a damn good spellcaster and can REALLY make up for what you lose on his path. Seriously, he's more potent than the MC (mostly), it just sucks that certain spells require other stats to be high as well.