How do I deal with this nasty coworker/head of another department?

Look for another job. For some reason, the Senior Manager doesn't like you and has it in for you. He is only going to make it worse for you in the long run. If he can, he will give you so much work you are buried and then he will sack you for being inefficient. And as for your gutless boss, he has made it clear he can't or won't stick up for you. So, keep on smiling, do your best, and find that other job now (preferably unionized!). Good luck!