If your players are like my players at some point they are going to rile the Technic League up enough that they are going to send bounty hunters. This trio – whose true name is unknown – is perfect for the job. Initially I am just using a standard Ogre-Magi from the Monster Manual. Power-up them as needed. If they are outnumbered by the party I would add legendary actions as well. They should be able to provide a challenge to the party no matter at what level they are encountered (if at all). Inspiration should be obvious.
THE ONI BROTHERS
The Technic League has many contacts in the world of bounty hunters and they tend to use the right tool for the right job. So when the Technic League needs someone eliminated with a show of force they send the ominous Oni Brothers to do the job. They tend to cause massive property damage and cause heavy loss of life when they pursue a target – which is exactly the way the League wants it – maximising terror. The brothers fight in their Oni form when combat starts though they may use their shape change ability for reconnaissance or infiltration or some other subterfuge. No one actually knows if they are brothers or not or what they even call themselves but thats the term used by those in the Nimmerian bounty hunter profession.
The leader is a standard Oni from the MM pg. 239. However Green wears O-Yori (as plate, bringing his AC up to 18) and wields a katana which acts as a greatsword.
As a standard Oni from the MM. However Red wears O-Yori (as plate, bringing his AC up to 18) and wields a naginata (treat as glaive) with such force he can knock targets back.
Brute. When Red Oni attacks with a melee attack he attacks with such force that he inflicts an additional die of damage (included).
Naginata. Melee Weapon Attack: +7 to hit, reach 10ft., one target.
Hit: 20 (3d10 + 4) slashing damage, and if the target is medium size or smaller they must make a DC 15 Strength saving throw or be knocked 10 ft away and fall prone.
As a reaction if a target is knocked prone by his naginata attack the Red Oni may advance 10ft and attack the same target again or another target within range.
As a standard Oni from the MM. However Blue wears O-Yori (as plate, bringing his AC up to 18) and wields a rocket launcher and a machine gun in addition to a katana attack (as greatsword, replaces glaive attack). If there are no enemies within 10 ft. of Blue when he attacks with the rocket launcher he may use a bonus action to fire the machine gun in the same round.
Rocket Launcher (1 per long rest). Ranged Weapon Attack: +6 to hit, range 600 ft. , multiple targets.
Hit: Each creature within 20 feet of an exploding rocket must make a DC 15 Dexterity saving throw, taking 5d6 explosive (force and fire) damage on a failed save, or half as much damage on a successful one. Objects take double damage.
Machine Gun Burst (Recharge 5-6). Blue can spray a 30-foot cube area. Each creature in that cube must make a DC 15 Dexterity saving throw, taking 49 (11d8) piercing damage on a failed save, or half as much damage on a successful one. Objects take double damage.