Class Points and Skill Points
The concept of learning abilities for your characters in Xenosaga II, is completely different from it's predecessor. So I'll start off by explaining this. There are a total of 4 skill levels, which are then divided into 8 classes (Class A - Class H). Though Level 4 only has 4 classes.
Skill Levels that are accessible to you are marked with the word "PASS", otherwise the selection will be grayed out. Now in order to learn a skill, you'll need to have sufficient "Class Points" (CPt) to unlock the class where the said skill is located. Then use the required amount of "Skill Points" (SPt) to learn the ability itself.
New skill levels become accessible, once you've learned all the skills in any class within the previous level. Class Points
are gained by defeating bosses, learning all the skills within a single class, or by using "Class Upgrade" items. Skill Points
are a lot more common than Class Points, and are gained from defeating bosses as well as normal enemies, or by using "Skill Upgrade" items.
The Event Slot is displayed at the lower-right corner of your screen. The sequence in which the slots are showed follow this order: CTC -> BST -> SKL -> "Random Slot".
CTC - Chances of landing a critical hit increases by 50%.
BST - Increases charge rate of boost gauge.
SKL - Increases the amount of Skill Points received from enemies (x2, x4 or x10).
ETR - Ether damage and healing increases by 50%
TC - (Turn Cancel) Target loses turn when damaged by an attack.
RV - Target's boost gauge charges when attacked.
NON - No effect.
Zones and Breaking
Knowing how to take advantage of enemy weak zones, is the key to winning battles in this game. You'll get wiped out pretty often (even by normal enemies) if you don't get a hang of this entire concept early. So to those who aren't sure how zones work, pay attention.
First off, enemies are categorized into 3 types: Mechanical (M), Biological (B), and Gnosis (G).
There are 3 types of zones: A, B, and C, and is displayed at the upper-left corner of your screen. You are allowed to execute 2 consecutive PHYSICAL attacks using the square and triangle buttons, while only 1 ETHER-type attack with the O button. Also be sure to remember this:
The O, Square, and Triangle buttons target the A, B, and C zones respectively.
Now if you managed to exploit an enemy's weak zone, (the letters will be shown in red) you'll be able to "Break" them. Thus allowing you to deal more damage. (x1.5) For example, a U-TIC Soldier's weak zone is BB so in this case I'll attack with  twice, to break him.
Some enemies however require more than 2 button combinations to break (usually bosses). Therefore you'll need to use 2 characters to accomplish this task. Here's how to break an enemy with a weak zone of BCBB:
Have Character A attack with  and Triangle (BC), then boost Character B and have him attack with Square twice (BB). Take note that Character B must be boosted BEFORE Character A completes his attack sequence in order for this whole thing to work.
Occasionally, a successful break earns you a "Break Bonus". This gives you a stock. The other thing that must be mentioned is the AIR and DOWN effect, which doubles the amount of damage received by your target. Air or Down can be executed in 3 steps:
Stock up your characters to at least 2. (A full stock is more effective)
Break your opponent.
Press O repeatedly (this will depend on how many stocks you have).
Once that's done, keep boosting your characters to keep the effect active for as long as possible. To determine which characters can cause Air or Down, look at the bottom- left corner of the screen. The arrow points upward for AIR, and downward for DOWN.
Stocking and Boosting
The Stock Gauge is located at the top of your screen, just below a character's EP value. Press X during battle to open the command menu, and choose STOCK. This adds a stock to your Stock Gauge. (3 max) Having a full gauge allows a character to execute 5 consecutive attacks.
The Boost Gauge is located at the lower-right corner of the screen. Press R1 or R2 buttons to open the Boost Window and press the designated button to boost a character. You need a value of at least 1 on your Boost Gauge in order to use this function. Also remember that only characters whose icons aren't showing at bottom-right corner of the screen can be boosted.
Double Attacks and Ether Combos
A double attack is performed by 2 cooperating characters, and is much more powerful than any other single character attack. But the requirements needed to execute one, are no less difficult to attain. To perform a double attack:
Stock up both characters A & B to the amount required for the attack. Now on Character A's turn, boost Character B. Then with character A, open the command menu and choose "DOUBLE". From there, pick the attack you want to use. Remember that a Double Attack for a pair of characters, can only be used once per battle.
Ether Combos require 2 characters to execute. So of course you'll have to use boost. In order to form a combo, both the current and boosted characters must have the same ether skill. Next choose ETHER from the command menu, pick your desired skill and press TRIANGLE. A new window should appear if you can form a combo from it, otherwise pick a different ether skill.