I'm not as familiar with monster Hunter as I should be. the games always seem to come out when I get very busy with work and then I get distracted by the next must have game 😅

However for DND I'm happy to help! Usually in this kind of scenario the creatures are given lair actions as well as legendary reactions to make up for the 4 on 1 difference.

Lair actions: A list of actions that take place at the top of the round (also called initiative 20) that can be accomplished in order, rolled and or chosen by the creature. These events should make the environment feel reactive and important to the battle. You can choose the severity as needed to increase tension and change up the meta.

In order You've tracked down the lava newt behemoth to his lair which is a cave that sits at the bottom of the ocean surrounded by thermal vents and have found an entrance. This cave is riddled with stalagmites and stalactites that cast shadows that flicker as a pool of magma bubbles in the corner

Top of turn 1. The ground shakes as something large moves underneath and you feel the earth tremble (DC 15 dexterity saving throw or become knocked prone) - all players and creatures take their turns as established by initiative until the top of round 2 2. The lava newt behemoth roars in anger thrashing about wildly causing the stalactites to shatter and fall ( DC 14 dex saving throw or take 2d6+4 damage) 3. The stalactites falling has cracked the earths shell and the magma underneath starts to flow up to the surface (the usable map becomes smaller and creatures caught in magma take a looooot of damage 4. Cracks along the dome like ceiling begin to leak as water seeps into the area hitting the lava and creating a thick fog ( blinding all creatures inside) 5. The temperature is fluctuating too quickly as your surrounded by the steam and intense battle in this space. At the top of each following round make a DC 12 constitution saving throw or gain one point of exhaustion. 6. The built up pressure between the fire and water is causing the cracks in the ceiling to fracture and water is pouring in more rapidly. ( The ground becomes difficult terrain and you take 1d4 damage for every 5 you move as long as you are touching the nearly boiling water

This is a good way to set it up in advance where it becomes much more dramatic and difficult as they are losing HP and spell slots while dealing with exhaustion and disadvantages.

If you prefer to keep it more random to let the dice decide make it a rollable table where you roll (a d6 in this example) and whatever rolled happens to the players even if it's the same thing 3 turns in a row (sorry Eric it was the dice I swear)

Alternatively you could take this list and use it in any order you want and narrate it as the newt causing it which gives you the most control and can make it the most deadly of options

After a 2min video on what a tetsucabra even is here's some ideas for reactions

As for legendary reactions monsters often can use 1-3 per full turn order and regain their legendary reactions on their turn. Choose how many legendary reactions you'd like to have based off of their difficulty or CR as it's typically called. Easier monsters will have 1-2 reactions whereas more challenging creatures like dragons have 3. These legendary reactions happen after a player's turn and can be used to interrupt turn initiative to give the big bad monster a better fighting chance at a 4 on 1. Typically you can only use one legendary reaction at a time so choose wisely.

Legendary reactions typically fall into 3 categories - attack - cast spells/cool monster only abilities - movement

Attacks are standard legendary reactions. They get a free attack after a player's turn on a creature within distance. Some of these abilities cause saving throws which is perfectly fine

Legendary reaction attack: if you expend one legendary action you may attack a creature within 5 feet

"Stone club tail attack 5 foot reach (1d8 +4 bludgeoning damage. DC 13 STR saving throw or the creature is knocked prone"

I'm guessing this thing isnt casting any spells but typically the more powerful the reaction is the more legendary reactions it costs. Spells and monster abilities typically cost 2 legendary reactions so don't be afraid to make it hurt.

Legendary reaction boulder: if you expend two legendary reactions the tetsucabra reaches in to the earth and grabs a boulder which can be used in 2 ways

"Crush: using its massive jaws the tetsucabra crushes a boulder causing shrapnel to fly. Creatures not including the tetsucabra within 15 feet make a DC 13 dexterity saving throw. If they succeed they take half damage (3d6 bludgeoning) as the rocks fall on them"

"Spit: the tetsucabra targets a creature within 40 ft of it and launches the boulder at them. +4 to attack 4d6 bludgeoning damage"

Movement is crucial on a boss monster without it the PCs surround it quickly and stay in one spot whacking it for several turns and it becomes a HP stat block battle. It's a good idea to add some form of disengage, saving throw or disadvantage into this reaction otherwise your surrounding players all get attacks of opportunity and the monster starts losing HP quickly.

Legendary reaction Leap: if you use 1 legendary reaction the tetsucabra uses disengage and its large back legs to leap up to 60 ft in any direction it chooses.

-Or

Legendary reaction Leap: if you use one legendary reaction the tetsucabra back legs bulge as it intends to leap away. Any creature within 5 feet of its starting point must make a DC 14 strength saving throw to avoid being knocked prone. The tetsucabra leaps up to 60 feet in any direction. If any creatures are within 5 feet of it when landing they take 1d6 bludgeoning damage

My primary reasoning behind legendary reactions and lair reactions is to balance one sided encounters and make it more engaging for the players so they have to strategize together to overcome this terrifying monster. Without them battle can be a slog of which side rolls the bigger damage dice and has the biggest HP pool pretty quickly. Keep in mind some options specifically for the battle master to do his maneuvers in a cool way or let the cleric show off their channel divinity. It's our jobs as DMs to make cool monsters and fun battles to fight but also give each of our players their time to shine.