I'm sorry you ran into people like that. Obviously, not everyone is like that.
Here are some tips for a beginning gladiator. Most of these only apply when you are in dungeons (which start at level 15 if you're following the main quest).
- Your enmity combo is Fast Blade -> Savage Blade (and later on, you'll get Rage of Halone as the third part of the combo). Use this as your primary means of attacking in dungeons. Do not use the Riot Blade combo even if it does slightly more damage. The difference is minor and keeping aggro is important.
- Pull with Shield Lob (you get it from your level 15 class quest). This has a 3x enmity multiplier, meaning it's three times as good as opening with Fast Blade for getting that initial enmity.
- Never pull with just Provoke. Provoke sets your enmity to the highest enmity in your party +1, so it is essentially wasted when used to pull and the first DPS to hit the target will steal aggro from you. That said, Provoke has a much longer range than Shield Lob. If you really need to pull something that's out of Shield Lob range (like, a patrolling enemy), Provoke to aggro, then Shield Lob when they get close enough.
- After you pull a group of two or more enemies with Shield Lob, run into the middle of them and use Flash. This will get you initial aggro on all of the enemies you're fighting.
- In the party list, to the left of people's names, are meters. These tell you how much enmity each person in your party has on your selected target. Whomever's bar is full should have an A next to their class icon indicating that they have aggro and the monster will attack them.
- When you're fighting a group of enemies, be sure to cycle through them regularly to see how full your party's enmity meters are on each. Remember that your job isn't to deal damage, but to keep aggro on everything. Do this by finding which monster you're closest to losing aggro on due to another party member's enmity bar filling up, and comboing that monster with your enmity combo.
- An easier way to keep aggro on a group of enemies is to just flash between each combo. This is not as efficient and drains your MP fast, but it's easier to do.
- When fighting 5+ enemies at once, just spam flash. You probably won't be able to keep aggro on all of them, but when you are fighting this many enemies, it usually means they're really weak and die fast.
- Mark enemies. I don't know the PS4 controls, but you should be able to assign the marks (the yellow 1, 2, 3 in octagons being the most important) to your hotbar. Mark the target you want dead first as 1, and so on. Don't bother marking the last enemy - a majority of people will understand that the unmarked one comes last. Make sure you get initial aggro on the one marked 1, as your DPSs will build enmity fast on that one.
- Newer CNJs don't realize just how much enmity Regen generates. If you go into a fight with Regen on you, the first time it ticks, your healer will probably pull aggro. You can cancel a buff by rightclicking on it (no idea how with the PS4), and you should cancel Regen before pulling at low levels if it's on you.
- Face enemies away from the party. Several enemies, and most bosses, have cleaving attacks that hit everything in an arc in front of them. Many enemies and bosses also have AoEs that they point in front of them. By facing them away from the party, these attacks will only have a chance of hitting you. To face them away, after you throw your shield lob in, run through and past them before stopping and turning around to fight them.
- Use CDs ("Cooldowns" - the buffs that take a while before you can use them again) liberally. They recharge pretty fast (most in 60-120 seconds) and are an important part of damage mitigation.
- Fairly early into the game, you unlock the Armory System, which gives you the ability to change your class at will. When you can, switch to MRD and level it up to level 2. This gives you the cross-class skill "Foresight" which is a defense-raising CD. To equip cross-class skills, open your skill menu (P on the computer, no idea on the PS4), and go to the cross-class skill section. You get one cross-class skill slot every 5 levels as a base class (like GLA), and every 10 levels as a Job (like PLD). For leveling (not dungeons), you might want to consider also getting the skill Feint from Lancer level 2. You can't use it later in the game when you become a PLD, but it can really help in the low levels.
Hope this helps!