a lot of the weapons, tank or human are a lot weaker despite their high attack power. For example, the 220mm gaia main cannon, fully upgraded has an attack power of 960, the highest amount of all usable weapons in the game, yet it only does a measly 40 points of damage on Cyber wall, and more often than not gets deflected on Noah.Yet at times it can do up to 1000 points of damage. Either the damage rating is far lower than a typical rpg, or the game is heavily depending on random number generator.
The wall just has a very high defense. The 40 damage would be normal hits and the 1000 damage is probably from criticals. To improve accuracy intstall a c-unit with high accuracy. The damage rating is also dependant on your driving skill, so a powerful gun wont do much damage if your level is low.