When you enter the Plains of Eidolon you must know what you are going to face. Essentially there are two enemy groups. On the one hand there are the Grineer occupation troops the Tusk detachment. On the other hand you have the Sentients who still roam the plains at night, the Eidolon Teralyst and Eidolon Vombalysts. Many of you may not be prepared to face the last ones, but here you are going to find what you need to defeat them when the time comes.
Good luck, Tenno!