Died a couple of times early game, finally figured out what seems to be the optimal strat.
The burn pegs burn the falling orbs and the freeze pegs freeze them long enough for the burn to kill them. Eventually other chutes start dispensing orbs, but by that time you can start buying turrets to shoot the non-burnt ones
First and third row (except for the electric peg which I just bought because I thought it would be an area-of-effect cloud, although considering what the other pegs do it makes sense that it doesn't) are all level two, but I believe it's probably more cost efficient just to save the money on healing
Great game!