You're just fighting FOEs you're not ready to go up against. Giant's Ruins can really fuck you up if you don't know what's the gimmick of the FOEs, though Golem isn't that hard. The Rebel Wolves in the 5th Labyrinth are just weirdly balanced and the fire attack makes it much stronger than the other FOEs in the labyrinth. It's one of those FOEs where a Protector really helps. And the Bounding Beast you're not even ready to take until high level 30s.
Bounding Beast can be taken around level 25 if you know what you are doing. And by know what you are doing, I mean Para, Blind and Arm bind, with enough damage to kill before you've burned those too many times. It is definitely a harder fight that you can't feel bad about ignoring for a long while.
Still, I'm finding the balance on Nexus to be a bit weird. I'm in the 5th Lab too, and I've felt that FOEs, bosses and regular encounters feel really all over the place. But I also feel that each classes balance is all over the place too. Like everyone says, Sovereign and Hero are easily the top two. I feel like a lot of other classes suffer from weird choices though. Between skill cost, distrobution and anyone not in heavy armor being really squishy, it feels a bit more awkward to navigate. I haven't played with all of the classes yet (Missing Ronin, Harbinger, Arcanist, Farmer, Imperial and Zodiac) though, and I am on Heroic(Expert), so that may explain some things.
Landshark, and to a lesser extend, Gunner feel really TP limited early on. Gunner has much better options to handle it, and ramp up on TP much faster, but linkshark feels really medicore early on. Their skills cost too much to use for a whole fight, but links aren't good until you get them up in level. Improved Link is hard to manage as a buff this time around and costs a lot. You're left in a weird situation, where you either feed tp items (Hamao at this point) to them, or watch them burn out really quick. Even if you go for their physical builds, 8 tp a shot on Double Strike is a lot for their small tp pool.Gunner feels a lot more fair, since their TP pool is larger, they get TP Up, and the results from their attacks are pretty respectable.
Then their are classes like Pug and Highlander, who are really just starved on points early on. It always feels like you are playing a lesser / far from complete version of themselves. Highlander starts to feel right around Vet, but Pug is still pretty point starved. On the flip side, I feel Ninja falls into this when jumping into Vet. Too many skills, not a lot of points, and most importantly, they all feel lesser than they should a lot of the way through. Sovereign comes close to this, but manages to feel better to use at half-steps, likely because there are fewer skills you feel like you need at any point (limited to 3 buffs anyway at a time). Hero also dodges this by having a lot of skills that are respectable at low investment.
Oddly enough, I think that Survivalist is the most balanced feeling class right now. Good variety of skills, useful at lower investments, supportive stuff, and no skill level feels overloaded. Shogun is a close second (more an issue of being expensive), and Nightseeker being third (useful ailments, stronger attacks early on, but proficiency and other passives feel pretty heavy on investment).
Medic feels like it always does. The question of, what do I do when I have nothing to heal.
War Magus feels awkward, but I need to give it another shot with Nightseeker. The skills and cost are on point for an attacker. The heals are pretty weak though.
For reference, I was running two parties of
Lands / Hero
Sovereign / Gunner / Ninja
Shogun (V) / Highlander / Pug
Medic / Survivalist
to start, but have shifted a few times. War Magus was bounced around a few times, but never made it as a regular on any specific team.
Right now I'm on
Shogun (V) / Highlander / Nightseeker
Sovereign / Gunner
(Still working on this one. Link focus make it a bit tough)
Lands / Hero / (Pug / WM / Medic)
Survivalist / Ninja