I had Merric heal/barrier on 95% of every turn in my current playthrough. Here are my turncounts:
Chapter 1: 16 turns
Chapter 2: 12 turns
Chapter 3: 18 turns
Chapter 4: 14 turns
Chapter 5: 11 turns
Chapter 6: 11 turns
Chapter 6x: 12 turns
Chapter 7: 9 turns
Chapter 8: 6 turns
Chapter 9: 11 turns
Levels at the end of Chapter 9:
Marth 8.94: 23 HP, 11 Str, 12 Spd, 9 Def
Shiida 12.34: 30 HP, 6 Str, 20 Spd, 9 Def
Cain 12.27: 29 HP, 11 Str, 13 Spd, 10 Def (C Swords)
Barst 15.02: 35 HP, 16 Str, 13 Spd, 11 Def
Cord 12.93: 23 HP, 13 Str, 15 Spd, 6 Def (C Bows)
Lena 11.19: 17 HP, 2 Mag, 9 Spd, 3 Def
Merric 10.54: 27 HP, 4 Mag, 13 Spd, 6 Def
Sedgar 3.54: 31 HP, 10 Str, 5 Spd, 17 Def
Mekkah has similar results, and a similar level for Merric at the end of Chapter 10.
Playing efficiently really cuts down on his level.
PS: I realize that Sedgar is a bit underlevelled.