1-hit Mode: 1. Click on the weapon 2. Click on the desired weapon 3. Type in the level of sharpness (0 if you don't want it enchanted) It will now tell you how many hearts of damage it does. 4. Repeat for armour It will now tell you how much damage the armour absorbs 5. (Optional) Click on the stars in the top left to enable/disable critical hits 6. (Optional) Click on "New Effect" to add a potion effect such as strength or weakness Finally it will tell you how much damage your weapon, with the potion effects, will do against the armour. Battle Mode: Fairly Similar to 1-hit mode, but it will simulate a battle between 2 players, 1. Click on the weapon and the armour to choose it 2. Change the Number of hits per second you want the player to have (Optional) Click on "New Effect" to add a potion effect to your player. 3. Click on the "Player 1" to change players 4. Choose Player 2's weapon, armour and hits per second (and potion effect if you want) 5. Click go to start the battle. If you do not choose the weapons and armour, it will be leather armour and stone swords.
thanks @Splodgey changed sharpness to 999 limit