It would be better to say, "Every summer (no need to specify you're taking about a season), it gets very hot, boiling even.".

The simplest way to communicate your point is to say (and be careful of context): "Every summer, it gets boiling hot!" That said, you could use a wordier version of the sentence for artistic purposes (a poem or limerick), such as the version suggested by Sergio Henriquez.

I would even say that it's boiling. "That" in this sentence refers to the fact that it's boiling. The first sentence is correct. Hope this helps. :)