about leveling up bonuses
should I just use them for one stat?
Like Remilia purely atk, Reimu magic etc.
Most people like to focus on one stat, but it really doesn't matter since level up bonuses are inferior to skillpoint bonuses. So basically whatever you choose, it can be offset by whatever you decide to invest your skillpoints into. If you want to be a real efficiency perfectionist, I suppose you can find out what the most expensive stat to raise via skillpoints is for that particular character (but is still a useful stat) and focus on that, but that's just kinda weird. Just don't spend it on sp ever, you can spend skillpoints on sp, and at some point in the game, you'll find yourself never needing more sp, you'll never run out unless an enemy casts destroy magic or something...Some characters are exceptions though, remi and chen come to mind.
also, when do I get more tanks :V
Remi is cool I guess, China is a pure wall and Youmu does one attack and she's out of juice.
This is a big flaw IMO, there is not enough variety in tanks. China is what I call a slot 1 tank, a tank that can survive in the first slot reliably. Remi is one too early game, but eventually she is probably best considered a slot 2 tank, Youmu is inferior to remi in this regard. Many characters can be considered a slot 1 tank in very specific situations, even patchy for example, but situational scenarios aside, It's pretty much just China and 10F tank. 6F girl has good hp and defens,e but absolutely horrid mnd, so, she can be a good slot 1 tank provided the enemy doesn't have a big single target/row attack magic nuke, Wriggle is also a kinda-slot 2 tank, umm.. Some people like 7F girl as a "hp tank". She really isn't a proper tank, she's just good cuz her hp is great (both her def and mnd suck), so generally she's useful for late game bosses who have no threat aside from attacks which ignore defense completely, etc.
Other things you should know...
Many of these you probably already do know:
switching costs 1 tp, when you switch a character out, it's as if they are still on the battlefield spamming "focus" the entire time, so in a boss fight, if you need a particular character to regain sp, it's often better to switch them than to focus them, focus is only better when you need that person out because they can take a hit better than any potential replacement or something.. They also retain all their buffs (and debuffs though) while switched out. So you wont have them wasting away uselessly while you're at 0 sp.
I've read some sources that say the people in the front 4 get 100% exp, the other 8 in the back get 90%, and everyone in the mansion gets 80%. My own personal experience says that all 12 of your party members get 100%, regardless of who is in the front or back. Maybe this changed with a specific version or the plus disk. I haven't really bothered to compare to people in the mansion.
You may want to use the wiki to research how to get some characters. There's no shame in not knowing that you need to fight 250 battles with 4 different but specific characters to get another character. There are only a few cases like this but it's good to know in advance. No, there is no way to tell how many battles you've fought. As long as you use everyone relatively equally until the point where you have more than 12 party members, you've probably met the criteria already for all of those joinables except the 12F girl. She's the only one that requires a battlecount for characters that you may not even have in your party at all ever.
Row attacks deal 100% damage to one target (normally left to right, but sometimes giant mofos in the middle count as the first), 80% to the next, 40%, then 20%.
The game doesn't really REQUIRE grinding until 16F boss/18f Boss, it's just pretty challenging if you don't grind beforehand, note that some bosses (Youmu is the only case that comes to mind for me, but I've heard of stories about 12F boss using their "IWIN" nuke early too) can cheese you if you have bad luck and they decide to perform an unfair combo on you when you are behind on level.
Status resists (statdown,poi,par,etc) are multiplied by 3. So having a par resistance of 20 actually means you resist it 60% of the time (it's actually more more often than not since most par attacks don't have a 100% rate of success, it subtracts the chance, not divides).
Bosses with the same graphic as the 7F boss are all very intimidating in their opening attack. That's as bad as it gets though, survive that gracefully, and the fight should be cake. Feel free to re-itimize/formation just for that attack. Squishier characters should survive mid-way thru the fight provided you buff them/debuff the boss.
durr.. umm.. I think that's it.