You know, time after time I see people getting frustrated over the Junction system, unable to understand it. Yet the very premise of it revolves around three simple rules;

1. The more of a number of spells you have, the more a stat will go up
2. The "higher level" a spell, the more a stat will go up
3. Certain spells will increase certain stats more than others

This inevitably leads to two conclusions. First, that spells should be constantly refined/drawn as needed throughout the game, and should ideally be at 100. Secondly, it's worth experimenting to see what effect different spells have on different stats.

When it all comes down to it, there's not much difference between the stats of the characters. It's best to use as many +stat boost abilities upon levelling up to gain the best possible levels. Since you can't have all five equipped (that's HP, Strength, Magic, Vitality, and Spirit), you have to choose. I personally have the core stats; Strength, Magic, Vitality and Spirit. HP, at level 100, can be easily junctioned to 9999. At level 100, provided you've had a fair amount of +Strength level-ups, you'll easily be able to reach 255, even with the very basic spells, such as the elemental -aga's or even lower in some cases.