How can I get the Stalker to spawn?
Solution 1:
The Stalker spawning is completely random, it can happen to almost anyone at any time.
The reason I say 'almost everyone' is because you need to have killed at least one boss since the last time you were Stalked before the stalker can enter one of your games.
The first time I saw the stalker I was playing by myself, and I was relatively new to the game. I doubt I had any last hits on any bosses at that time, though I had beaten some by playing online with others who did the killing for me.
So other than the initial condition of having to beat at least one boss in your entire career, the stalker can spawn at any time, in any game mode, on any player.