Juvia is the 9th strongest wizard in Fairy Tail. Besides being a crazy stalker she's also crazy strong. Looking back she was part of the Element Four and depending by her emotions she is able to make herself better and stronger that's also when she met Gray which made Juvia stronger.
Juvia has many awesome attacks such as: Water Nebula, Water Cannon, Water Slicer, Water Lock, and Water Jigsaw. Since she is made out of water when something pierces her it just goes through her body plus she is able to make rain causing her to become more powerful. In the Galuna S-class trials Juvia and Erza met Meredy who was determined to kill Gray at first Meredy was ahead in the battle but after Juvia heard her attempt to kill Gray she went into full Love Psycho mode making her stronger and fought for her love for Gray. In the Grand Magic Games she also used her love for Gray and made a special water attack using her Second Origin.
In the Tartaros Arc. Juvia has to fine one of the Nine Demons of Zeref who is Keyes. Juvia fighting Keyes was difficult because had the ability to bring back dead people and use them as puppets which sadly one of them happened to be Gray's Father. Juvia knowing this didn't want to break Grays heart but almost died by not doing so but in the end Juvia defeated Keyes believing in the power of love and how Gray and Silver's love for each other will reach them. I love how Juvia believes in the power of love and that if there is love in our life we must fight to live for it which made me love Juvia in the end teaching us that love prevails no matter what.