Marcia has the worst combat ability in the game. I wholeheartedly encourage you to level WuJie or someone better to L100 first, and unlock all ten dungeons to unlock better items.
First thing out of the way; you absolutely do NOT want to try levelling Marcia first, nor by fighting similiar level monsters. Her defence is above average, but her attack power is completely pathetic. This would be fine if her skills weren't also completely worthless. (Two of them increase the damage you take for goodness sake! >_<)
The best way to level her up by far is to gather a bunch of money and purchase recipes from the Mystery Man at the Market until you unlock the L90 or L100 poison recipes, and then stockpile Genesis Seeds and Energy Liquids from the materials seller who is also at the Market. 2x of either will allow you to make 1x L90/L100 Poison potion in the forge at the Guild.
Poison potions deal fixed damage to whatever enemy you are currently fighting when used. In other words, you'll be able to instantkill up to three enemies in the final dungeon if you use at least one or the other poison when you meet an enemy. It's the fastest way to level up Marcia and anyone else without fixed damage attacks like WuJie! :3 And before she's L50+ you can level her super quickly by just buying the L80 poison potions and sending her off into the 7th dungeon, the Cave.
Just poison three enemies to death, and either let them kill you (Seems to be no penalty apart from ending that run early.) or keep escaping for the chance of finding extra items in a chest further ahead.