I've been in your shoes. People like to say "just play," but if you can only make a tiny handful of vanquishes in turf, then it's not terribly helpful advice. Private matches vs. AI worked well for me during that period.
Sunflowers level fast if you focus on healing and revives. G&G is a good mode for her because you can play by the base. Try to go for xp from assists, base defense vanquishes, saved an ally, that sort of thing. You can actually earn xp pretty quickly with sunflower by just going around the backyard keeping all the weeds healed up and not shooting many enemies so you don't escalate the game to heroes and champions.
I liked learning how to cactus playing on great white north. Scientists make good enemies. Cactus eventually went from being my worst class to one that I now main. Citrons I did mostly in ops, but you can also earn a lot of xp playing him in vanquish type games with sunflowers as team mates.
When your AI games end, look to see how many xp you are earning per minute. You will figure out which game modes and maps are the best earners for you for the various characters.