FEATURES
Use comments in a troop page to assign that troop a condition for it to appear in random encounters. This condition can be a switch on/off, a variable above a certain number or if there are a certain number of items in the inventory.
For advanced users, they can use javascript to determine the condition also.
VERSION 1.2 – RPG Maker MV
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Galv's RPG Maker Scripts & Plugins 
Updated to version 1.1 fixing a bug with random encounters not using their designated regions
Updated to version 1.2 – can now use multiple condition tags. Also added a way to use script code to determine conditions.
Just what I needed!
Thank you, will set a variable for level and be able to add more encounters with harder enemies as the players level-up. Awesome !