Cellion |
The harder Robot Dragon in the appendix on page 82 is indeed CR7. This makes it a very challenging fight for a 4th level party of 4 characters.
To work out what CR is appropriate for your group, you should check page 28 of the beginner's box GM guide. This walks you through the steps of determining your party's Average Party Level, or "APL". You add up all the levels of people in your party, then divide by the number of party members. Since you have fewer than 4 people in the party, you should subtract 1 from the result (My rule of thumb is that going down from 3 to 2 players is such a big drop in party strength that you should actually subtract 2 from the APL instead). For a 3rd level party of 2 players, that would be an APL of 2 (or 1 if following my rule of thumb).
Then look at the Encounter difficulty table in Step 2. This tells you how high of a CR in total the encounter should have. While the table uses words like "average" "hard" and "epic", my rule of thumb is:
In your example, your party has an APL of 2, so a CR7 encounter would be an APL+5 encounter! Nigh impossible. An appropriate "hard" encounter for two level 3 characters would be a single CR3 or 4 creature, or two CR2 creatures. A boss encounter might be a single CR4 or 5 creature.