After trying to play the game for free, I can attest that it is impossible to be competitive. However, after sinking too much money into it, I can also attest that spending money will only net you the same three and four star low caliber heroes, regardless of how many summons one pays for. In addition, the random raids often put ones heroes against a team so much higher that there is no way of beating them. Move on to a game more rewarding, or settle for killing time playing for free.