Defeat all in-game bosses first.

Step 1
Defeat squid guy, also called moon lord. This needs to be done right after you defeat him


Step 2
After you defeat squid guy :
Say ''Come fight me, Shrek!'' 100 times into the chat, so shrek has a chance to spawn


Step 3
After shrek being summoned, the fight will begin and you have to defeat him


Step 4
When you defeat Shrek boss, the Shrek Npc will spawn



The end. Hope this helps you on how to summon shrek boss and npc
Keep in mind that the fight is pretty hard. Good luck on your fights