Don't half-ass this project or I'll fire you.
I don't want to half-ass this project or I'll be fired.
I half-assed this project and was fired.
There are some similar phrases such as:
Some good words not as deep into slang would be
I don't want to [be] unmotivated [on] this project or I'll be fired.
Don't slack [on] this project or I'll fire you.
I [did] shody [work on] this project and was fired.
I use "BS," which stands for "bull-sh!t." In my high school, we students often talked of BS-ing our English papers....and basically every other assignment. BS-ing something more or less means making sure everything takes the proper form (but the substance may well be ridiculous) and having neither time nor inclination to bother with it any further.
Sounds like what you're looking for. :)
I don't want to do this project half-heartedly. I want to put due effort into this project. I want this project to succeed.
I would suggest "play at":
I don't want to play at the project or I will be fired.