I leveled to 50 solely by doing standart quests not a single flashpoint, no bonus series and mostly no heroic quests, no PvP.
Only thing I did extra was Space Combat every mission 3x while it was yellow-orange-red(1st time when it appeared, then immediately after as the usual 2 mission pack and then next day before starting quests)
And I also did my companion quests as I leveled = no 5XP rewards but 10k XP rewards for most dialogues. You need to improve your companion affection during leveling and you get maybe 2 levels just by doing that.(If you have Ashara buy 75x lvl 1 green gift at fleet)
Leveled doing yellow quests all the time but cannot say it was struggle. Did 49->50 on Belsavis bonus so maybe I wasn't accurate - Correlia left me on 49 sorry.
PS: Helps a lot to group as you get 100% more XP from bonuses in a 4man party.