What is a good way to tackle difficult technical questions during graduate interview
Relax it is the most important thing; if you are stressed it will show. Also people ask tough questions not only to see if you get the answer right, but also see how you get to the answer, and to see how you respond to the answer you gave being challenged and questioned.

Also If you don't know something or are unsure but think you could have a stab at it say so. Some people may not appreciate this but other really do. I have left a couple of interviews (I got the role) where people commented on my honesty. The person asking the question generally knows the answer, so don't BS them.

Talk of people knowing the answer, I worked with one guy who used to want to know the answer to esoteric or difficult technical stuff, so asked the same question to at least 2 candidate, and had a set of 5/6 questions. They would design chunks of the system in the interview, and then he'd pick the best guy. He had no idea what the answer should be, but figured if two people gave the same answer it was probably in the ballpark.

As I said at the start relax, one way to get a second to think is take a sip of water before answering. It allows you to get your thoughts in order after the shock of the questions. Also ask for clarification, even if you understand the whole question, again 5 seconds can be enough to pull you head back to the task, and dredge up those critical facts.