Hello Minmixmitch! Welcome to the hive workshop.
To get your hero past level 5 when fighting Neutral Hostile this is what you need to do:
In the world editor along the top click the "advanced" button then scroll down to "gameplay constants" which is the 4th option down. Scroll down until you reach about 2/3 of the way and look for the field that says "Hero XP Gained - Creep Reduction Table." Make sure that the box that says "Use Custom Gameplay Constants" is checked then double click on the field stated above. Currently there are 5 integers here that state how much xp is reduced through the levels. For xp to stop at level 10 just add more values using the add integer button. Now, if you want there to be no reduction at all and just have the same amount of xp per creep kill no matter what level you are delete all of the values in this field then add just 1 and make it 100. This meens that there will be no reduction. Hope that i helped.
Welcome to The Hive,
-Inverted