The main thing Assassins need to be aware of is their mana.

Ideally, you need to use  Walk in the Shadows (R) as often as possible to help regenerate your mana, and to empower your other abilities.
You do not need to stay cloaked to gain the added mana regeneration, as it persists for the duration after break.

If you continue to have mana issues, you can leave your abilities at a lower then optimal level; it only affects their static damage increase, and not the modifiers.

Attribute points should be spent in approximately a 1 Strength : 4 Agility ratio to begin with. The goal is to have no more then 24,981hp at level 200. With Ares, you're aiming for approximately 249 Strength, 3236 Agility, and 15 Intelligence

For your familiar skills, you want to invest in the 5% damage buff Ferocity and your choice of 75 Strength/Agility from Gift of Dexterity / Endurance, ensuring you don't go over the recommended HP.

At max level, the core setup is                            /      , combined with your choice of Olympian Relic;

1.    Ares Bloodthirsty Spear - Objectively best, combines damage, survivability, and can supplement damage further using Ares' Sword of Command on your familiar.

2.      Towering Shield of Athena - With TSoA and Assassin's naturally high armor, you will take virtually no physical damage, similar to the Midas' Band preventing incoming magical damage to become virtually unstoppable, but with reduced damage output.

3.      Thundering Hammer of Zeus - Good damage & group buffs

4.      Blazing Sword of Helios - Good damage, outdated build.

>>Blanket Disclaimer: You, and every other player will need items from all crafting professions so there is no 'best' profession for any given class. If you are selfishly looking for personal enhancements, Alchemy is the only profession whereby you can't bring items you crafted through to another game, so this might be the best choice on a personal level, but increases dependency on others for crafting<<