Reduce our problem to a known np-complete problem
The hint tells you pretty much everything:

Consider an arbitrary graph $G_{2}$ on $n$ vertices. 
Also per the suggestion of the hint, let $G_1$ be the complete graph on $k$ vertices, for some $k \le n$. 

Is $G_{1}$ is isomorphic to a subgraph of $G_{2}$? 


If yes, what does that tell you about $G_{2}$?
How can you use that to solve the max-clique problem?