How much health enemies have is already scaled based on party size (damage dealt and received by enemies might also be scaled, but I haven't been keeping track), so scaling the number of enemies would be redundant, as well as inconsistent. How do you scale down the number of enemies when it's one Trojan?

Addressing the topic of you getting slaughtered, what does the danger level read for you? By danger level, I mean the colored dot next to the mini-map. If it is red or purple, you need better equipment, no debate. If it's green or blue, better equipment will help some, but you'll need to work on your tactics if you're getting slaughtered. If the danger level is blank and you're getting slaughtered, no amount of equipment upgrading will fix that.

Now we come to what will be most important for you: tactics.
Tactics and skill are what will get you through the deeper levels and bosses.
Tactics and skill are not the same thing; tactics are your game plan for taking out monsters, while skill is your ability to pull it off. Skill comes with experience and practice, but without a good understanding of tactics skill is useless.

One of the most important tactics is that of not getting hurt. If ever possible, dodge your opponent's attacks. That means understanding your opponent's attacks so that you know when to dodge and how. If you can't dodge, shield. But time spent shielding isn't time spent attacking, yet time spent dead isn't time spent attacking either, so shield rather than take damage.
And yet, you never have to dodge, shield or take damage if your opponent never gets an attack out. The last strike of a sword combo or a sword's charge attack will make most monsters cancel their attack. If by continuously attacking you can ensure that a monster never has a chance to attack, you have have an advantage and should press it. If you can corner them, you may be able to use this against multiple enemies simultaneously.
However monsters can also also cause you to cancel an attack, so don't use this tactic against more enemies than you can manage at once.

If you are faced with such a situation, back up and separate the enemies into manageable groups. If you don't have enough room, such as in a battle arena, then run and attack when possible, but don't let yourself get mobbed. If you can, use your charge attack; if you only have time for one attack, make it count. If there are too many ranged attacks for you to avoid them, keep your shield up and only take it down to attack.

And yes, tactics and skill ARE very important. I once did a battle arena and Roarmulus Twins with a party member wearing starter armor. (He had his three star armor equipped as costume. Don't ask.) and yet he held his own. You may be wondering how he survived in Teir 2 with 0 star armor. It's quite simple, in theory. If you never get hit, then it doesn't matter how well your armor holds up when you get hit. The part that takes tactics and a whole lot of skill is not getting hit.