In that case, try to not rush in. Part of what helps is knowing what your limit is. I know it will take a while for improvement to be seen but I think mindset plays a huge key in improvement. Think of it like this. Lucina's attacks deal a good amount of damage and she can rack up damage consistently. Therefore, you shouldn't worry about get KO's as soon as possible. Instead, take your time and think about what your actions will entail. Lucina can be aggressive, but in order to play offensive, you must have a defensive approach, something that isn't always easy. But remember, you'd shouldn't make it your goal to take out your opponent as fast as you can with Lucina, make it your goal to rack up damage while keeping them out of your space. It'll take time, but don't get frustrated, you'll get there. If you need anymore advice there are tons of people who can help. Also I recommend looking at Art of Marth from Izaw, he covers Lucina there as well, and the principals of Marth he explains in that video mostly translate to Lucina as well, so it should be able to help out a bit if you haven't seen it.