Hi. For Conjurer (not White Mage) these are, to me, some of the more useful cross-class abilities.

I would definitely get Pugilist to Level 6 for Second Wind, and Gladiator to Level 22 for Sentinel, which is still useful as a White Mage.

Thaumaturge magic is very useful when playing as Conjurer, but you cannot use them if you switch to White Mage.

Second Wind (Pugilist Level 6) -- Instantly restores HP without using your MP.

Sentinel (Gladiator L22) -- Temporarily reduces damage taken but increases enmity. Useful for surviving certain strong attacks or if you accidentally get hate from one or more monsters.

Thunder (Thaumaturge L1) -- Deals lightning damage.

Necrogenesis (Thaumaturge L6) -- Restores HP when next attack spell is cast.

Fire (Thaumaturge L10) -- Deals fire damage to target and enemies near it.

Darkseal (Thaumaturge L14) -- Increases magic accuracy of next cast.

Resonance (Thaumaturge L22) -- Increases range of next cast.

Sanguine Rite (Thaumaturge L30) -- Restores MP in proportion to damage taken. Casts cannot be interrupted while effect is active.

Fira (Thaumaturge L34) -- Deals fire damage to target and enemies near it.


Other possibly useful abilities:
Decoy (Archer L2) -- Consumes MP to evade a single ranged or magic attack.
Quelling Strike (Archer L22) -- Reduces enmity and increases TP generated by next attack.
Chameleon (Archer L42) -- Reduces enmity.
Shield Bash (Gladiator L18) -- Delivers a melee attack with your shield. Chance to inflict Stun. (if you use a wand + shield combination and happen to be within melee range)