Hello, hoangboss06! You have cult spell that needn't mana to be used, for exemple Poisonous Spit (cult Fear), or Accurate Thrust (Valor) etc. This kind of spells is available for every Cult and it requires only some points of energy to be used and no mana. Also there are some elixirs like "snowball". It doesn't require any mana or energy and can inflict damage it-self.
gigibusuk, hello! You can get diamonds easily for winning in adept tournements, receiving medals or spinning for free 4 times a day and for rank in the rating. You can also find diamonds in cave. If you need more, you can buy some. Besides, you can wait for discount and buy diamonds X5.
towler4, hello! To get rid of monsters that you don't want to deal with, you can chase them away for some energy, or just go to the next island to find the monsters you need. The monser level depend on the island level, so you can't see 1lvl monster in 8lvl island. And even if you can reset all monsters, you will see the same strong monsters instead of matazuu or spirit of the blade. But the game is always changing, so it is possible that we'll see some new monsters soon.