Originally Posted by Myrdin

Gerbera doesnt really matter in this case, since she is a Slice type girl and thus will not compete for the magic manyus. As for leveling, always prefer 6* as your primary target, since they get the most out of lvl ups, due to the raw stat increase per level.
As such, go for Leucoryne first, and only start on Mirabilis once Leucoryne is past her Evolution (I usually take it easy once I Evo, and do 2 more upgrades, usually netting me around lvl 35-40. After that the girl be strong enough and you can shift your focus to the other one)

Leucocoryne is a 5*...

Anyways, Mirabilis is decent, but I wouldn't make her a priority. 15% attack for 2 squad members is meh and 80% chance of counter is decent. Event girls outside of the first 3 (Lavender, Edelweiss and Mountain Lily) generally have slightly watered down versions of the abilities that their gacha counterparts have, to make up for the fact they're MUCH easier to get maxed out (4 equips, level 5 skill). Her skill has a 28% chance to do x2 damage to 2 enemies

Leucocoryne is really useful after evolution for pest node heavy maps due to her evo ability being 10% heal on pest node for team. Her base ability increases her attack by 10% every time you use Solar Blast on a stage (with any team), stacking up to 30% at 3 uses. That ability's usefulness depends on how much you use SB, obviously. Her skill is 24-34% chance of dealing 2.3x damage to a single enemy.

Personally, unless you have Edelweiss or another gacha girl that heals on pest node, I'd focus on getting LC evolved first, but it's up to you.