Charmander can breathe fire, which allows them to use a wide range of Fire-type attacks. However, the strength of these are very closely connected to the strength and health of the Charmander, since it draws on the life force which fuels the flame on its tail to fuel the flames of its attacks. With time, it can learn powerful Fire-type techniques like Flamethrower and Fire Blast.Its most notable feature is the flame burning on the tip of its tail, which is there even at birth. It is said that if this flame goes out, the Pokémon dies. -Bulbapedia
pikachu is still a good choice. set its moves to Electric based attacks by deselecting the others. then set your charmander's attacks to something like: Metal claw or Scratch. if your hp goes down, use a fire based attack. since your hp is low, blaze will kick in.
Another hint: if your pikachu knows thunder wave, use it.