what bosses can a assassin farm?

Farm bosses which are equal or lower in lvl than your assassin, because sicarius.
Don't farm bosses you can't kill in your assault time.
Prioritize bosses whose drops sell for a good amount so you're not wasting your time, and can be sold relatively fast so you don't have to wait forever to achieve profit.

Vida

For farming, I suggest start with Sofys (divine tower, level 6). She is neutral element (most assassins usually own an eventail) and very low health. You should be able to kill her within 5 seconds alone. 

With mq/apos, yes. I personally suggest doing prioritizing that over farming if you can (or farm for some money if you need to pay people to help your mq). Unlocking Apo allows you to overcap Int, which is extremely important. You cannot farm well if your mp pool is so low/mpc is so high that you need a ton of potions. 

Disregarding how fast you farm bosses, you will probably level at bosses more easily than you can at yardes, so unlocking apo allows you to start leveling that. Solo boss farms (especially divine tower bosses, because they get stronger by the number of people in party) is an extremely efficient way to level.

You probably can't 1 hit nommars, guaranteed cannot 1 hit megas, and will die often at yardes, so killing bosses is the most secure way of exp. If you fight easier mobs, the amount of exp and spina you gain is not worth the amount of mp, time, and money for potions you are spending. Even if you can level efficiently at mobs, bosses are still better, because you are solving both the problem of money and levels by doing 1 action.