This would be my method, though there may be others with a better solution:
1) Have the heroes obtain the item as a Key Item without that has no traits of any kind. This will be what the player sees in their inventory. "Excalibur"
2) Create a skill that will be used in the battle behind the scenes without the player knowing. "Excalibur Skill"
- When this skill is used it will execute your action text. Under Using Message in Database/Skill, "(User name) rushes forward with Excalibur in hand!"
3) Set a condition on the Database/Troop screen for your battle with the boss. After the boss reaches a certain amount of health, or an amount of turns that you find acceptable pass, use the Force Action command and make your hero use the Excalibur Skill. "King Arthur rushes forward with Excalibur in hand!"
4) Once the skill goes off, just after the forced action you can use a Show Text command, choose a picture and put in your dialogue. "King Arthur: Take this, Mordred!"
5) At the end of the condition, select change enemy HP, select the boss, and then subtract whatever amount you see fit as long as it kills them. Make sure to check the "Allow Knockout" box. This will give all xp, gold, etc. that would've gone with killing that enemy.
6) The same event that linked to the boss fight can be used to continue on to the rest of your cutscene. After the "Battle Processing" event that started the fight, just continue directly below it on the same event page. The game will run right through them.
I think that was clear enough, but if it wasn't I'd be more than happy to elaborate. Good luck!