When Doffing look for the very high XP missions that involve diplomatic escort and Dominion related activities, usually found in the Beta Ursae sector where DS9 is.
You just need a strong starting cadre of security officers and diplomats, which you will get semi-automatically as you level the characters. Try not to waste time on any mission that gives less than 150 commendation points.
Even without mirror event or fleet grinding, you should be able to gain one level every episode mission + PvE event + a few duty officer missions completing.
You can also gain massive XP during the Borg Invasion sector events, look out for the Red Alert icon while cruising along in sector space. You got to click it fast but its worth it. You can gain many thousands of experience points from just watching a few Borg ships explode (other players do most of the work for you even if you're at low level and the XP gains do not require full mission participation).
STF Flight Instructor since Early 2012. Newbies are the reason why STO lives and breathes today. Do not discriminate.