1) Pair Chrom with Sumia or Cordelia 2) Have the Pegasus Knight fly up to Tharja 3) Switch to Chrom 4) ??? 5) Profit
There is no need to bait her.
Also, FE AI has always been terrible. I once had two of Trabant's Wyvern squads commit ritualistic suicide by attacking my Ced at 0% in chapter 9 of FE4.
I know my problem with that was my Chrom wasn't invincible enough too survive everything around Tharja. In that case, baiting was quite nice. --- "Hey I know! I'll use my trusty frying pan.. as a drying pan!" - Brock