Dude the answer is right in front of you!
Create a variable "main character's level"= main actor's level (all levels usually are around this).
with the boss event have it change to a different page within the event when the variable hits between certain numbers. Use a different "troop" as the boss fight with the same graphics per level section of the variable which controls the page.
This way the player can choose whether to grind or not, at times bosses will be more challenging. and as you progress you will need less and less event pages because your characters will (unless you give no XP) level up. This system will be a lot like Final Fantasy 8. there should even be a way to make normal enemies do this: The easiest way is to completely copy the maps you are using and auto teleport after area transition where the teleport to location is variable dependent on the "main character's level" variable discussed earlier.
It has been done before but it might be what you need to achieve your goals... this makes grinding pointless... unless enemies that only appear when you are at high levels for the area drop items that will unlock something like gear, a unique dungeon, a poodle sidekick with psychic powers...ect.
I don't like the whole "this makes grinding pointless" systems people invent. It's another act of condescension from game programmer to player. Dead Island takes this approach and nowhere in the history of gaming is it more obvious why you do NOT do it that way. Meanwhile, games like Dead Rising do it right.
The absolute ONLY reason to level up in Dead Island is to obtain more skillpoints for your skill tree. Where's the downside in that? It's actually beneficial to do as LITTLE leveling up as possible because as you go along in the game, the weapons get more powerful than the zombies for the area. If I pick up an "endgame" weapon at level 10 and I'm in the jungle, I am essentially murdering every zombie I run across in a single hit. Meanwhile, if I'd taken every quest and explored every bit of the island before getting to the Jungle portion of the game (third act), the zombies would be mauling me to death CONSTANTLY because my level is so high and their damage increases exponentially in order to compensate for my own level. It easily makes act 3 one of the most difficult portions of the game just because of the sheer number of zombies (including the standard variety that at the beginning of the game could be killed with a single hit of the weakest weapon) that can dish out extremely massive amounts of damage. The system then becomes "is the skill point bonus really WORTH the zombies getting stronger?" The answer is "no". Their system effectively renders level ups useless and worthless. Especially when at around level 20, you're already noticing how much harder everything is to kill (often taking 4 or more swings of a weapon to kill, or 3 shotgun shells to the head for a basic enemy).
Meanwhile, Dead Rising does it absolutely beautifully. Grind if you want. Get to level 50 before the end of the game if you want. What does leveling up do? It grants you skills and pips in attributes to let you more easily navigate the absolutely congested halls of the game. The driving principle of the game is then "what am I carrying in my inventory and how skilled am I?" instead of "what level am I?". Levels help, sure. They help immensely. But, a single zombie can still munch two bars of health off of you if you aren't careful of how you maneuver or play the game. This is also nothing to say about how much the Psychos (who are mini-bosses, or actual bosses) take off your health. But, gaining levels lets you do more damage to them as well. Sometimes, it even limits how much damage they can do to you. But, if you're not carrying the right equipment with you (like a decent weapon and a few food items to heal), then it doesn't matter what level you are. The only thing that keeps you from taking damage at all is good maneuvering and dodging skills.
Basically, if you want to eliminate "grind" from your game at all, just eliminate your level up system entirely. If there's no levels to gain for possible rewards, players can't and won't grind them. It's also a more clever way to handle it other than "as you get stronger, enemies get stronger". I don't know about anyone else, but when I figure out that's what a system is doing, I start ignoring 90% of the content just to keep my level low enough to absolutely murder everything I run across with ease. If you're going to punish me for gaining a level, I'm not going to gain levels. I'll beat your game at level 4 with God equipment that destroys your bosses, because they're scaled to my level.
I'm sorry, I just think the whole concept of it is fairly silly. Grind should be
optional for players. If I want to grind mats or weapons or something, I'll gain levels for it. If I want to gain a few extra levels so I can bypass most of the combat in the next section or two and enjoy the story a bit, I should be able to do that. But, if all you're doing with your game is "I want to tell a really great story and have people engaged instead of gaining levels", then you should eliminate the source of the problem to begin with. Get rid of level ups. They are going to serve no purpose except to annoy your players and force them to play the game in the most non-intuitive way possible.
In my game, that's all I'm doing. I'm just trying to tell a story. As such, I changed my level up system entirely. All leveling up does in my game is provide shortcuts, teammates, or access to a couple areas. I wanted to emphasize story and exploration in my game, so I give out equipment and stats as rewards for completing Quests. Granted, that runs me into one of the same problems of "the party can't overlevel for an area", but I've solved that by adding in some shops that sell these stats for extremely high prices. These stats also in the early game typically only come in quantities of 1 or 2. It isn't until late game that you're getting them in quantities of 5 or 10.
Now, keep in mind, that's just how I chose to handle the issue. You can be creative and handle it a different way. Just, if you have a level up system, it needs to provide great tangible rewards that make the players WANT to level up instead of discouraging it. Otherwise, what's the point in having it?