Secondary professions for an Assassin

Assassins are highly aggressive, focused damage dealers, better for hit and run skirmishes than drawn out tanking or back-line sieges.

Always remember, when choosing secondary skills, that native skills often have better synergy with the profession's purpose, interact better with the profession's other skills, and benefit from runes.

Assassin/Warrior

 * Warrior stances are useful to increase attack speed. Flurry will reduce weapon damage.  Frenzy is especially dangerous because of Assassin's comparatively low AL.  And Tiger Stance's long recharge makes it useful only during spikes, not for pressure.
 * Assassins wielding an axe can take advantage of the higher critical damage from axes and Axe Mastery skills that benefit from critical strikes.

Assassin/Ranger

 * Barrage damage can be improved with high critical strikes attributes, adding bonus damage to attacks, energy management, and effects from skills like Critical Eye.
 * Disrupting Accuracy in combination with high critical strikes and skills like Way of the Master and/or Critical Eye make nearly every shot an interrupt. Combine with IAS for even greater brutality.  Keen Arrow also synergizes with the assassin's high crit rate well.

Assassin/Monk

 * Assassins can take self-heal skills such as Live Vicariously and Vigorous Spirit with Critical Defenses to provide decent self-heal and survival.

Assassin/Necromancer

 * Rigor Mortis can bypass blocking to ensure your chain lands.
 * Necromancer hexes synergize with "Black" attack skills and with various Deadly Arts skills.

Assassin/Mesmer

 * Arcane Echo can be used to keep up Shadow Form.
 * Perma farmers find Channeling useful when farming for energy management.

Assassin/Elementalist

 * Shock can knock down foes quickly, which combines well with skills such as Falling Spider.
 * Conjures can be used to trigger "Golden attacks"
 * Glyph of Swiftness can be used to reduce the recharge of Shadow Form to an infinitely manageable skill.

Assassin/Ritualist

 * Weapon spells (such as Warmonger's Weapon) can aid an Assassin in battle and are immune to disenchantment.
 * Splinter Weapon can be used to do damage to nearby foes.

Assassin/Paragon

 * A spear-wielding Assassin with a high critical strikes attribute and skills such as Way of the Master and Critical Eye will be able to deal substantial damage, due to a spear's fairly high maximum damage and Vicious Attack.
 * Even with no points in Command, "Go for the Eyes!" provides a significantly higher chance to land a critical strike.

Assassin/Dervish

 * Critical Strikes used in conjunction with a scythe can create massive critical hits.
 * Some Dervish enchantments can help the Assassin survive longer in battle, or increase damage output.