Secondary professions for an Assassin

Assassin/Warrior

 * Due to the high dagger attack speed, this combination generates lots of adrenaline. Skills such as "Fear Me!" are good for putting this adrenaline to use.
 * Assassins do not have any attack speed enhancing stances, and can thus greatly benefit from Warrior skills such as Flurry for DPS, and Tiger Stance for spikes.

Assassin/Ranger

 * Barrage damage can be improved with high critical strikes attributes, adding bonus damage to attacks, energy bonus, and effects from skills like Critical Eye.
 * Disrupting Accuracy can be combined with critical-enhancing skills such as Way of the Assassin to provide constant interrupts.

Assassin/Elementalist

 * The knockdown from Shock can drive an attack chain, though it is no longer as popular as other skills such as hexes to drive the attack chain.
 * The Conjure line of enchantments can increase damage with the assassin's high attack rate.

Assassin/Mesmer

 * Arcane Echo can be used to chain Shadow Form when running.

Assassin/Monk

 * The Protection Prayers skill Mending Touch is a good spell to remove risky conditions.

Assassin/Necromancer

 * Curses hex spells, particularly Parasitic Bond, can be used to drive many attack chains.

Assassin/Ritualist

 * Sparing use of weapon spells such as Warmonger's Weapon can aid an Assassin in battle, although weapon spamming is best left to Ritualist primaries.

Assassin/Paragon

 * This combination offers little benefit to an Assassin primary.

Assassin/Dervish

 * The Assassin Dervish can use Critical Strikes to greatly improve the Critical Hit rate with a scythe.