Soulkeep - How Can Heartgiver Really Help?

I am enjoying soulkeep so much and all I want to do is to keep playing and although progress is not so fast but I enjoy getting more experience about cards and fiends and I think now I do not miss anything about the game especially in league one but to be true may be spells I am not so familiar of most of it as I do not try all of it and I use low mana ones.

So I have rented some legendary spells to have them a try in my current level which is level 4 and I know that I am using spells so good as high waves fiends that they come together they need such as explosion to eliminate them all and also to benefit from them being together in a narrow range as towers my not be such effective to kill them all while passing in front of them.

Today I want to use my legendary spell which I have rented and to see how effective it is in the battles as you know that epic and legendary cards have a good feature as it can be used at level 2 and in this case you my heartgiver spell that is legendary one will give me more hearts.

I have used HeartGiver from a legendary type and it is level 2 so it gave me 4 hearts, I have used it twice first time I have used it after wave 1 as I have lost about 8 hearts and it compensate me with 4 hearts so I am back to 8 and feel I am still in the battle, Second time I have used it at the start of the battle before playing as I want to know if it can be used or not and it is effectively adds 4 hearts to me to start the battle with 16 hearts instead of 12.