Hm...not sure what to say for the 3x3 attack. Sword has a skill Winged Slayer (Weapon Skill Level of 10) that targets a 3x3 area; likewise, Fist has Big Bang (WSL of 20) though it has Fire Element attached to it. Also, spells can eventually get to a 3x3 target area through a combination of Staff Skill and Spell Skill Level.
*Weapon Skill Level is raised when you attack. The easiest method, personally, is to just go to 3-4 and attack the Wall of Meat since they never attack. Incidentally, due to this you can combine them to get more EXP and never worry about dying.
Staff Mastery (spoilered for length!)
[spoiler]Staff Mastery Chart - Every level adds 3% to power.
0 - +0 Range, +0 Area. 100% Power.
1 - +0 Range, +0 Area. 103% Power.
2 - +1 Range, +0 Area. 106% Power.
3 - +1 Range, +1 Area. 109% Power.
4 - +1 Range, +1 Area. 112% Power.
5 - +1 Range, +1 Area. 115% Power.
6 - +2 Range, +1 Area. 118% Power.
7 - +2 Range, +2 Area. 121% Power.
8 - +2 Range, +2 Area. 124% Power.
9 - +2 Range, +2 Area. 127% Power.
10 - +3 Range, +2 Area. 130% Power.
11 - +3 Range, +3 Area. 133% Power.
12 - +3 Range, +3 Area. 136% Power.
13 - +3 Range, +3 Area. 139% Power.
14 - +3 Range, +3 Area. 142% Power.
15 - +4 Range, +3 Area. 145% Power.
16 - +4 Range, +4 Area. 148% Power.
+4 Range and Area is the Limit. Power % can go all the way to 200%.[/spoiler]
As for combining an enemy too strong for you ... meh. Merely do a long (i.e., 10 person!!) combo to raise the damage. Also, abuse things like Target Lock and Braveheart (5 casts is the max raise, though it decays per turn at a rate of about 5%. Each cast is +20%) to get your stats up.
Finally, Armsmasters and Statisticians (1900 and 300 cap, respectively) are pretty much vital for leveling. Armsmasters increase the rate of which your Weapon Skill Level increases by 1% per Armsmaster (so a 1900 is +1900% or 20 times the base rate) and Statistician does the same for EXP (so 300 is +300% or 4 times the base). They do not stack beyond this so all you really need is a 950 Armsmaster and 150 Statistician (unsubdued) to be subdued so you get the right value. This is accomplished by buying items with them and subduing the innocent in the Item World, moving it to the next item, combining them and then repeating the process. Takes awhile, sure, but the amount of time saved in the end more than makes up for the setup time.
Also, WSL has a direct impact on stats derived from weapons:
1 + (WeaponSkillLevel * .05) = Weapon Stat Multiplier
So, for example, a WSL of 20 nets:
1 + (20 * .05) = 1 + (1) = x2
And a WSL of 255 (the max) nets:
1 + (255 * .05) = 1 + (12.75) = x13.75
Armor multiplier is level based and maxes out at x14.99 at Level 9999. I forget the values at various levels before that and I don't think I have my old D1 notes anymore
Edit: As for naked level 1's one-shotting stuff ... most the enemies in the game have poor gear or low health barring certain stuff (Boss-labeled, Gatekeeper, Item Bosses, Baal) so that a fully reincarnated unit can probably one-shot a lot of enemies naked since you can get up to ~1.2M ATK. Fully reincarnating (i..e, maxing both the Blue and Yellow Bars on the Reincarnation screen) takes <10 hours to do per character ... and really only takes so long due to having to watch that damn Winged Slayer animation.