here’s the not-so-good news: you can’t actually run it and see your enemies move around and shoot and stuff. XML by itself is really more of a data structure than a programming language than you might think. the XML simply holds the information for game entities and the actual rotmg game interprets that and turns it into a thing you can actually interact with. you can write up everything and make it good to go and stuff, but there’s no way you can know for sure that it actually works. there’s a reason I’m the only person (that I know of) who’s ever written up all the XML for a whole dungeon idea that wasn’t actually working in UGC to put it in the game: it’s a ton of work for something you’ll never even know if it works. the only way you’ll ever know is if you do manage to get into the closed testing group, which most people think is impossible.
that said, if you choose to proceed down this harrowing road and conquer the trials that await you, Notepad++ is what I used. if you want to make things a touch easier for yourself than they were for me, make sure you get the XML plugin for Notepad++. it will auto-close tags and stuff for you and it will also tell you if there’s an XML structure issue that would prevent it from compiling.