Yeah multi build is the best thing to get in my opinion. Buildings start taking days to upgrade so being able to do more than one is hugely convenient.
Zera is also great. She's been on my team since I got her from the Christmas event. Her skill is kind of weedy, but she attacks so fast she can put out a lot of hurt. Girls also don't take any extra exp to level if they are higher rarity in this game, so she'll level fast enough.