Instead of starting the the map in local mode, or anywhere else, go to practice, and start it there.
That's how to get to practice mode, just in case you started playing yesterday, or didn't care to navigate around the menus for 12-16 seconds. Then all you need to do is select your character (it makes 0 difference on gameplay and does not matter who you pick), make sure you don't have a pet equipped (it can break things, especally if it's a non-ordinary pet that messes with things), and start the game with the map.
When ingame, you will still have access to the practice mode pause menu, and the something helpful is the time manipulation. You can slow the game down, to for example, dodge attacks easier. The option is changeable on the fly, so you don't need to worry about everything being slowed forever. The other options, such as hitboxes, have no effect on anything. especially since you should of started the game without anyone else, even CPUs.
There ya go. If you see this guide and you haven't played Rivals of Aether (somehow), I recommend you pick it up. It's pretty good. I mean, they modded deltarune into it. That's pretty cool.