I am fairly disappointed by the items available in the Christmas Event. I just do the 120 for Tactician Books so I can do more TBS (though not really). I learned that mercenaries that are not B rank or better deal pitiful damage at lvl 130+ skirmishes at least. This is a very short one because Inventor only has 1 skill and very few uses.
The Inventor is a Gun mercenary that has a special feature. He can equip machines in the place of where bullets for the Gun types go and use his Summon Machine skill to summon the machine to support you in combat. The machines can act as another Gun or Cannon type for additional stun capacity. Because of this, the Inventor has the greatest ability to stun compared to any mercenary. However, the Inventor has fairly low damage potential alone. Janissary has a far higher damage potential. If you run a stun build, the Inventor is vital because he adds a lot of stun pressure. Keep in mind that if the Inventor dies, the summon disappears as well.
The only skill that the Inventor can use is Summon Machine. The effects of the machine is determined by what you have equipped in his off hand slot where bullet normally goes. Damage from the machine is increased with a higher level Summon Machine. It is primarily used for additional stun pressure so the ballista is usually the preferred weapon. Some people like to use the catapult due to the potential to possibly hit 5 targets, but far more people use Guns. With Guns, it is a lot easier to stun a whole column at once compared to a Cannon which would not always hit a column. Summon Machine is useless in skirmishes because it becomes a cross shaped area damage skill rather than an extra unit that you can use.