A battle happens from the point you declare you wish to attack, and thus any effect that prevents cards from being activated at the point of a battle starts cannot be activated, see the first example below for how to write that kind of effect
Least of all this card back, which uses the aesthetics of a stage to paint a captivating back design
In Tempest, it was turn one (Mogg Fanatics, shadow creatures, and flyers), Invasion turn two (2/2 bears for every possible color combination plus several one- and two-mana apprentices and utility creatures), and Mirage turn three
Machoke's ability to learn a variety of moves, such as Karate Chop, Submission, and Dynamic Punch, further enhances its utility in battles
If this card is sent from the hand or field to the Graveyard
However, while it would be a fine inclusion, I've not seen any evidence that players are picking Rain of Gore up to play it in that deck