Strategies to avoid being underleveled as a F2P in SW:TOR?
I just recently started playing SWTOR as a F2P player. My experience so far has shown that as long as I complete all the side quests and heroics, I am usually a level or two higher than my current quests. Not much, to be sure, but it's enough to give me the edge I need, and I play a tank Jedi Guardian, solo except for the Heroics.
I don't PvP at all, so that's definitely not it. I run the Flashpoints once, mostly to see the content, but that's definitely a good way to pick up good gear if they run it multiple times when they're eligible for it. F2P players CAN do the heroics multiple times, so if they don't mind grouping, that would also be a good way to gain experience and potential gear upgrades.
If your friends are unable to beat a boss at the end of a sidequest, that means they may be focusing on the wrong aspects of a fight, or bringing the wrong companion with them to complement their playstyle.
They should also be keeping up with their equipment; quest rewards are important to keep you at the proper power level, and spend those commendations before you leave the planet entirely. Modded gear has a bit more complexity, but make sure your armoring, and hilt/barrels are the best you can get; they're what determine armor and damage.
I also started casually (2-3 hours a day during weekdays, maybe 5 or more during weekends) playing as F2P for about 2 months now and so far I have a Level 50 Sith Juggernaut with Columi Gear (bought an artifact authorization for a very cheap price in GTN), gearing towards BH/Rakata and a Level 32 Republic Commando.
Anyways, so far here is what I think the best way to maximize leveling up as a F2P player in SWTOR
As @fbueckert said, side quests are really helpful in gaining that extra 1 or 2 levels above your current main class quests. In addition to that, some of the side quests' stories in itself are sometimes interesting so I don't mind doing them
As much as possible do the bonus objectives on quests as they give some extra useful exp
Try to get all the quests on a single area as chances are some of the side quest objectives reside very close to the area to which your main class quest objectives are located
Try grouping with other people in the same planet. Not only are you able to do HEROIC quests which gives a ton of exp and some good gear, but you can also do main/side quests faster since a group's kills/gathers are counted as one therefore you will be able to do objectives faster. On a side note, avoid doing Heroics and Area Quests on your own, not only is this very hard, but this will cost more time and therefore will slow you down your leveling significantly
Do daily Flashpoint and PVP quests. These can be found on the fleet on the Supplies and Combat Training sections respectively and these gives a ton of exp.
If you have extra credits, level up your legacy perks in order to boost experience in the following subcategories
A. Class Quest Experience
B. Warzone Experience
C. Space Mission Experience
If you're asking what about Exploration Experience and Flashpoint Experience, well exploration really gives just a little bit of exp and will only be good for the first times that you explore places. Flashpoints on the other hand takes longer to do especially if you are grouped with less experienced people (one FP tends to last anywhere from 20 mins to 1 hour). I really suggest spamming flashpoints on the endgame where good gears drops from there, and not while leveling. Only do FPs for the sake of the daily rewards.
Save up some credits to buy yourself Major Experience Boosts on the GTN. This gives you a 25% exp boost on everything you do that gives exp for 3 hours and this is maintained even if your character dies or if you logout! These typically costs anywhere between 10-40k credits but it is really worth it. On the other hand do not buy specific exp boosts (warzone/FP/Class/Space Missions) as these will not stack and the overall credit cost for these is far more expensive than the generic major experience boost.
If groups for Heroics and main/side quests are not available on the planet you are on, try going for PVP experience. This, in my personal experience is the fastest way to level up. You do not have to win the PVP matches in order to gain exp (but it does give you a very large extra for performing very well). A typical match only lasts between 10-20 mins, even faster depending on the warzone and gives huge exp. In order to spam this, I buy myself one of those weekly warzone passes on the GTN for an average of 80-100k credits. This gives me unlimitted warzone participation instead of just 5 per week!