What are you looking surprised for? Of COURSE there's a big tough bastard here at the end. That's just how things work. I don't make the rules.

There are a lot of archetypes that are meant more for enemies than for players, but none so much as Boss. While not technically illegal for a player to take levels in, they will have a much harder time making proper use of the archetype than the powerful opponents hand-placed by the referee to challenge them.

Note for referees: The Boss archetype makes a creature comparatively much more powerful than its level would otherwise suggest. A Boss does not exist to defeat the players so much as to provide an interesting challenge to them, so try to design boss fights with that in mind.

Boss Break the rules when fighting solo.
caret-right Arena Home turf advantage.
caret-right Belligerence Boss attacks, optionally telegraphed.
caret-right Defiance Conditions don't stick.
caret-right Rusher Unlimited moves, wider choices.
caret-right Wellspring Make your own resources.

Boss

Utility plus-circleYou See A Bad Mutha: You gain up to 3 bonus actions you can take every round, called “boss actions”.

minus-circleHead For The Turn Around: Your boss actions have a number of limitations. You can only use a boss action at the end of an opponent's turn (after that opponent has used both of their actions). Boss actions are limited to making a movement or recovering (restore 1 point of lost Energy and attempt to remove one active condition). You can only make a movement using a boss action once per round.

minus-circlePaid the Cost to be the Boss: You gain no benefit from the Boss archetype or any of its abilities unless you are alone with no allies to back you up. Allies that you personally created/earned through the use of another archetype ability (such as Tamer, Necromancer, Overlord, Summoner, etc) are exempt from this limitation and can freely back you up in a fight.
Offense plus-circleFucking Lumbergh: You stress people out. Any creatures facing you in combat immediately gain 1 Stress (maximum once per session).

minus-circleSpongebob: Creatures with the Fool archetype are immune to this effect.

Arena

Utility plus-circlePlace Of Power: You've got a special lair called your “arena”. Your arena is a single dungeon node, but can be any size that makes you interesting to fight in (probably ranging from a smallish room on up to a cathedral/gymnasium). While you are fighting in your arena, you can make special arena actions as one of your boss actions.

minus-circlePreloaded: Your arena has a selection of three different arena actions it can grant. Pick from the below list or make up your own (of hopefully similar power). Some arena actions are reversible.

minus-circleMix It Up: You cannot use more than one arena action per round, nor can you use the same arena action two rounds in a row.
Arena Feature Boss Action
Elemental Energy Flash. Every combatant in the arena (including yourself) immediately gains a chosen elemental condition (such as Burning, Confused, or Irradiated). Choose which condition your arena inflicts ahead of time; you can't change it later.
Floodable Fill. Your arena has 3-5 different elevation levels. Every time you take this action, you cover the lowest uncovered one in shallow water (counts as difficult terrain) and any lower ones completely in deep water (requires swimming).

Drain. You can also take an action to drain the water in your arena, completely uncovering a shallow-water layer and making the highest deep-water layer into shallows instead.
Special. You can replace water with something much more dangerous, such as lava or acid. Chose one elemental archetype, such as Firebug (for fire) or Mordant (for acid). Wading or swimming in the substance automatically inflicts the relevant elemental condition and removes 1 point of Flesh per round from the poor bastards who get caught in it (including yourself). Creatures that are immune to the chosen elemental condition lose Energy instead of Vitality when exposed to the substance. This counts as two of your three arena abilities.
Recall Get Back There. Specify a single location in your arena. You return to it instantly no matter where you happened to be in the arena before regardless of distance or barriers in your path.
Sealable Lockdown. All the entrances and exits to your arena close on their own, preventing escape. They automatically open again if you are defeated. There might also be a clever way of getting them to open again while you're still alive (such as via an operation), but anybody trying will have to deal with you attacking them at the same time.

Open Up. If you already put your arena on lockdown, you can open it up as an action instead.
Sensors Reveal. You learn the locations of all hidden enemies. Until the end of your next turn, you may completely ignore the effects of both partial and total concealment.
Shortcut Open The Gate. Your arena is divided into two or more distinct areas that require a bit of work to travel between. You can create a shortcut that greatly eases travel between them (such as by opening a gate, lowering a drawbridge, activating a teleporter, or similar). Once the shortcut is active, both you and your foes can make use of it. The shortcut cannot be the only way to get around your arena, just by far the most convenient one.

Actually, No. You can also use an action to close off the shortcut again. If the shortcut involved has a creature currently standing on/in it when you remove it, they are harmlessly shunted off to the nearest available space.
Spawn Local Lackey. Your arena provides its own servitor creatures, and you can summon a new one with an arena action. The arena's servitors have one level for every 3 that you have (so if you are level 6-8, your arena's servitors are level 2 but once you hit level 9 your arena's servitors become level 3). Every arena servitor has identical combat ratings and abilities. Arena servitors are under your control and do not count as full allies, allowing you to continue to use Boss abilities while they are backing you up. You cannot have more than three arena servitors active at a time.

Belligerence

Offense plus-circleExtra Slam: You may make weapon attacks as boss actions. You may only do this once per round, even if you have access to multiple different weapon types.
Offense plus-circleTelegraph: Instead of using an attack normally, you can use a boss action to telegraph an attack instead. Telegraphed attacks do not land immediately like other attacks do. During any future turn, you can use a boss action to execute your telegraphed attack. Telegraphed attacks deal +5 damage.

minus-circleReserved: If you telegraph an attack, you cannot make any regular attacks with the weapon you're telegraphing with until the telegraphed attack actually gets used. If you choose to make a regular attack with a weapon that you previously set up a telegraph with, the telegraph is canceled and is wasted.

minus-circleAlways A Delay: You cannot set up a new telegraph in the same turn that you execute a previous one. Set up and execution are mutually exclusive boss actions.

minus-circleVisually Apparent: Your foes know when you've telegraphed an attack and which weapon you're using for it. This gives them a round to react.

Defiance

Protection plus-circleChumbawumba: Every time you gain a new condition, roll a trigger die. If the result is 7+, you instantly remove it before it has a chance to inconvenience you at all.

Rusher

Movement plus-circleComin' At You, Bro: You may make movement actions as boss actions multiple times per round if desired.
Utility plus-circleVersatility: You may make any action other than weapon attacks (or abilities that count as weapon attacks, such as using Tactician to order an ally to make a weapon attack) as a boss action.

minus-circleOnce Per: You cannot make any given action more than once per round. You can freely take different actions during the same round so long as you don't repeat yourself.

Wellspring

Utility plus-circleOMG HAX: You never run out of juice. You can use a boss action to grant yourself 1 of any resource (Provisions, Supplies, Mojo) up to once per round.

minus-circleFading: All resources gained via the Wellspring ability disappear at the end of the current battle. These resources cannot be used for any purpose other than powering attacks or abilities during the fight.