Your most general en safe options will probably be:
"Go for it."
"You can do it."
Crushing it is a common expression used when someone is doing their job particularly well, or exceeding all of their goals. Unlike the literal definition of the word “crush” (to destroy with force to the point of injury), “crushing it” has an extremely positive connotation.
In your case you could use:
"Go crush it."
Killing it has a similar meaning to crushing it, but it's probably wise to stay away from a phrase like "Go kill it".
In a more competitive setting this could work highly motivating for the receiver:
"Show them who's boss."
If you feel like the receiver isn't 100% confident about the task he/she is about to perform, for example when it's his/her first time, you could throw in a more supportive:
"You got this." And optionally add: ", this is your game."
For the final two options below, I think there's quite a 'risk' of the receiver not knowing what you are talking about, but I think they are technically correct. And if the receiver does know them they could be highly appreciated. But I would say they're your less 'safe' options.
If it's a task that has been on his to-do list for a while and he finally gathered the motivation to do it, you could refer to Mark Twain's (Briand Tracy has written an entire book about it):
"Go eat that frog."
If it's a task with which in the ideal outcome brings in money for the company and you want to make him/her feel like a valuable team player:
"Bring home the bacon."
Hope this helps.