I received a Slaking in a trade with pretty good ivs. Using Skill Swap, entraiment or something like this, could be possible to remove truant from Slaking? ( even in a single battle )
Yes
Truant cannot be overwritten by Simple Beam, Worry Seed, or Entrainment, but it can be overwritten by Skill Swap, Role Play, or Doodle, or disabled with Gastro Acid.
Truant can also be replaced by Mummy by making contact with Yamask-Cofagrigus. All of these would probably only work in Doubles,(with Doodle actually only working in Doubles)except Gastro Acid if predicted. Hope this helped!
Source
Source about Mummy
Yes, you can remove Truant from Slaking in battle