Your Difficulty Calculation is Incorrect

The XP numbers represent the lower end of each type, such that:

• 0-299: Trivial (easier than easy)

• 300-599: Easy

• 600-899: Medium

• 900-1599: Hard

• 1600+ - Deadly

Your encounter was therefore hard, not deadly.

Difficulty Comes in Depth

Any single encounter, unless ridiculously deadly, will not challenge a party if they are allowed to have a long rest between every encounter. Difficulty comes when many encounters slowly whittle down the party's resources.

Further Reading

On the topic of creating and running challenging combat encounters, you may wish to read the following articles by the Angry GM (bad language warning):

• Creating Combats Part 1

• Creating Combats Part 2

• Creating Combats Part 3

• Running Combats