What is Magmut's role in PvP ?
Magmut is a fairly self-sufficient raid boss; it can boost its own defenses and special attack using Madness Buff, Charcoal Wall and Block. Once set-up, Magmut threatens its opponents with its boosted Fire/Melee coverage, and is nearly impossible to take down thanks its traits, granting it a lot of HP recovery over the course of a game.
Its great traits, Physgulp and Specgulp, grants it 12% HP recovery every time it is hit by a physical of special attack, respectively. Along with extra recovery from Nutrition bar, this gives Magmut a fairly easy time setting up Madness Buff and can easily become impossible to defeat by ennemies that attack on its trait's side.
What kind of teams appreciate Magmut ?
Magmut's main issue is that it has to choose between its two awesome traits, because it will only regain extra HP on either physical or special moves. This makes external recovery options important so it can have an easier time facing damage that bypasses its trait. This makes Magmut a prime pick in raid boss teams, granting it both several recovery tools along with a quicker defensive setup, but it can also fit into midrange with some good allies.
Nidrasil is a great teammate in almost every composition because it can threaten water types with Toxic Ink, and its Spores move is one of the reasons Specgulp is prefered over Physgulp. With Nutrition Bar up, a single Spores can recover 19% HP immediately along with the 3 regen ticks, mitigating a lot of damage while Magmut is setting up Madness Buff. Spores is also an excellent tech move against Sleep-inducing opponents, as a Nidrasil that is faster than Magmut can break the Magmut's sleep status while recovering additional HP. Nidrasil can also bring Bark Shield to help for defensive set-up. Kinu and Pigepic are also excellent allies in any raid boss team, granting a lot of defensive boosts and extra recovery.
What is the best way to play Magmut ?
Magmut can safely lead if it's not facing direct threats as it can start setting up immediately, but it can swap in later into the match. Finding the right moment to setup Madness Buff is key. Use its allies to quickly ramp up defensive boosts. A simple Madness Buff -> Block rotation along with a Protector and a Sacrifice buff from Kinu can quickly skyrocket Magmut to +5 in both defenses.
Once setup, Magmut uses its remaining stamina to threaten opponents with powerful Soul Shouts and Fiery Souls, trying to hit physicals attackers first, as their attacks won't trigger the Specgulp recovery. Fiery Soul's priority and burn can also be something to consider when attacking, allowing you to outspeed and burn some slower Temtem to soften their attacks and get some extra chip damage against very bulky opponents.
If Magmut is safely walling both opponents has sufficient recovery from allies, you can also attempt another Madness Buff to further increase Magmut's offenses...but it may be overkill or risky in some situations.
What is the goal of this specific build ?
Magmut's main issue is its very low base Stamina, along with very costy attacking moves. That's why this particular spread runs a very huge Stamina investment. By reaching 83 total Stamina, Magmut benefits from extra regen each turn while also enabling a Soul Shout, Soul Shout, Fiery Soul rotation or a Madness Buff, Block, Soul Shout, Fiery Soul rotation at full stamina. Playing Magmut alongside reliable defensive buffs and extra recovery makes investing that much into Stamina viable while still keeping its bulk sufficient. HP is then maximized so Magmut gets as much recovery as possible from all sources (Nutrition Bar, Specgulp, Regen, Revitalize...) and the remaining TVs are split into defenses so Magmut ends up with a well balanced bulk, surviving OHKOs from most opponents.
Specgulp is prefered over Physgulp for team synergy reasons (Spores Nidrasil for extra recovery) and because the early Arbury metagame showcases a lot of powerful special attackers like Momo or Valash.
What can actually deal with Magmut ?
The main issue with dealing with Magmut is checking its trait. Once you know which attack type will trigger the extra HP recovery, you want to try pressuring it with attacks from the opposing type, while hitting its several weaknesses in the process. You could also break through its trait recovery with super-effective moves in the early turns, but don't count on that once its fully buffed up, as it will recover as much or even more HP that the damage you would deal to it. Sleep users is also a fairly decent option, making Magmut a sitting duck while to deal with its allies. Beware of anti-Sleep techs like Energy Drink Nidrasil however.