This mastery page is a pretty standard AP caster setup that I use for when I play most of my AP caster champions.
I put 21 points into the Offense tree to optimize my damage output. Let's go over them together.
: I put a point in Double-Edged Sword to help bring more damage to the table. It is basically a trade off where you deal more damage, but in return you take more damage. A good pick up when playing Cassiopeia since it gives her a boost in her already huge dps kit.
: I max out this Mastery skill for the 5% Cooldown Reduction. This means you will be able to use and spam your abilities more which is a great stat to have on a spammable champion like Cassiopeia.
: Mental Force when maxed gives us 16 Ability Power at level 18. Maxing this skill will give a nice boost in damage in the mid game stages where Cassiopeia damage capabilities really show.
: Arcane Mastery gives us 8 flat Ability Power meaning with just this skill and your Quints you are looking at 13 flat AP at level 1. More ability power means more damage which is great for any AP caster. Putting a point in this skill is a must.
: Executioner increases damage dealt to champions below 50% Health by 5%. Which basically means more damage ! A great skill to max that synergizes with Double-Edged Sword. Max this skill out to optimize your damage output.
: Archmage increases Ability Power by 5%. Not that big early on in the game but will be a huge boost when in combination with Rabandon's Deathcap which will increase Ability Power by 35% in total. This is also another must to optimize your damage output.
: Dangerous Game is a totally underrated mastery skill. To the common players it is just a waste of a mastery point. Who needs 5% Health and Mana restoration after killing an enemy. That's nothing. WRONG ! When you go all in against your opponent he will be also throwing everything he has. Most cases this will often lead to 1 side killing another but then dieing seconds later due to a last tick of ignite or minion aggro. Well this is where this mastery point shines. After killing your opponent is restores 5% of MISSING Health and Mana which means the lower your Health and Mana after killing an opponent the more it heals you for. This means instead of getting killed by the last tick of ignite or getting killed by an OP purple caster minion after killing your opponent it will heal you enough to survive the damage to come out on top. This mastery synergizes really well with a clutch level up.
: Arcane Blade gives your auto attack some bonus magic damage based on your Ability Power. This helps you with auto attack harassing as well as last hitting minions and hitting towers. A nice skill to put a point in.
: Devastating Strike is a new mastery that gives us 6% Armor and Magic Penetration when maxed. In combination with your Magic Penetration marks, this will boost your damage output on spells, as well as give a nice boost in damage to your auto attacks as well. This is a skill that is must to be maxed out.
: Havoc is the last skill in the Offense Mastery Tree. It gives us 3% increased damage which in unison with Double-Edged Sword and Executioner gives us a whooping total of 9.5% increased damage. With all these masteries taken in the Offense tree your damage output will be at it's maximum potential.
I put my remaining 9 points into the Utility Tree for some bonus utility and cooldowns. Let's go over them as well.
: Phasewalker reduces the casting time of Recall by 1 second. Doesn't seem like much but can be the difference of you being caught in a brush recalling and getting killed or making it just in the nick of time.
: Fleet of Foot gives us 1.5% Movement Speed increase when maxed which is basically 1 Movement Speed Quint. I max this over Meditation since 3 Mana regen is really nothing and it a stat that can be easily bought. Movement Speed is also useful and synergizes really well with your passive that we will later go through. Movement Speed is also useful throughout all stages of the game. It can be the difference of dodging a major skill shot CC or getting caught by it and dieing.
: Summoner's Insight reduces the cooldown of Summoner Spells by 10% when maxed. This is really crucial since Flash is a really important summoner spell to have up. If you enemy mid laner did not put a point into this skill, your flash and ignite will be up before his meaning you can go for a kill since his summoners wont be up.
: Alchemist is a nice little bonus utility spell to have. Increases duration of potions and elixers by 10% meaning more health and mana regeneration on potions as well as a little bit more time on your Elixers.
: Last but not least is Rune Affinity. This skill increases the duration of Shrine, Relic, Quest, and Neutral monster buffs by 20%. This means your blue buff and red buff will stay on your longer, as well as Baron. A really good utility skill to put your last point in. Help you farm more when having blue buff and spam your abilities more. Also having Baron buff for an extra 20% longer isn't the worst thing to have.