Well, you can have rematches with Gym Leaders in Emerald, so this is all 5 matches against Flannery (If you wanna have rematches and if my sources are correct)
To get the Heat Badge (And TM50):
Numel Lv24: Overheat, Take Down, Magnitude, Sunny Day
Slugma Lv24: Overheat, Smog, Light Screen, Sunny Day
Camerupt Lv26: Overheat, Tackle, Sunny Day, Attract
Torkoal Lv29: (White Herb) Overheat, Sunny Day, Body Slam, Attract
Round 2 (The rest are optional, like all rematches)
Magcargo Lv38: (White Herb) Overheat, Attract, Light Screen, Rock Slide
Ponyta Lv36: Flamethrower, Attract, Solarbeam, Bounce
Camerupt Lv38: (White Herb) Overheat, Sunny Day, Earthquake, Attract
Torkoal Lv40: (White Herb) Overheat, Sunny Day, Explosion, Attract
Round 3
Growlithe Lv41: Helping Hand, Flamethrower, Roar, Sunny Day
Magcargo Lv43: (White Herb) Overheat, Attract, Light Screen, Rock Slide
Ponyta Lv41: Flamethrower, Attract, Solarbeam, Bounce
Camerupt Lv43: (White Herb) Overheat, Sunny Day, Earthquake, Attract
Torkoal Lv45: (White Herb) Overheat, Sunny Day, Explosion, Attract
Round 4
Houndour Lv46: Roar, Solarbeam, Taunt, Sunny Day
Growlithe Lv46: Helping Hand, Flamethrower, Sunny Day, Roar
Magcargo Lv48: (White Herb) Overheat, Attract, Light Screen, Rock Slide
Rapidash Lv46: Flamethrower, Attract, Solarbeam, Bounce
Camerupt Lv48: (White Herb) Overheat, Sunny Day, Earthquake, Attract
Torkoal Lv50: (White Herb) Overheat, Sunny Day, Explosion, Attract
Round 5
Arcanine Lv51: Helping Hand, Flamethrower, Sunny Day, Roar
Magcargo Lv53: (White Herb) Overheat, Attract, Light Screen, Rock Slide
Houndoom Lv51: Roar, Solarbeam, Taunt, Sunny Day
Rapidash Lv51: Flamethrower, Attract, Solarbeam, Bounce
Camerupt Lv53: (White Herb) Overheat, Sunny Day, Earthquake, Attract
Torkoal Lv55: (White Herb) Overheat, Sunny Day, Explosion, Attract
Do you got all that?