Hm.
I'm just gonna list what I see on the top of my head and go from there:
Olivia is one of the most common dancers that many newbie teams tend to include to get that double action, so she's fine to include on your team. HOWEVER, in later stages, you'll still need a strong fire/red unit, and unless your team is based off the 'nuke everything with ranged/super strong units',
Olivia will not help in taking down any notable green units (specifically, Camilla, Hector, Julia, and Michaelis).
You may want to try leveling up Lon'qu if you find yourself with Olivia being the last unit on the team.Merric, the Wind mage, is fine. Just remember that if you plan on investing in him heavily for that sweet Excalibur skill (good against fliers, so you basically have a pseudo-archer on your team!), you should have a healer with
Rehabilitate (NOT Sakura; try using Wrys or Lissa and get them to 4 star) to help him in more serious binds.
A
healer is your best bet for now, given that your other archers (Niles, Virion) are too niche and weak to deal with most threats.
Shanna is... debatable.
Ideally, you want to find out strategies that synergize well when you're trying to mix and match heroes of different typings. Shanna reduces speed (which prevents your units from getting doubled), but her ATK is low and she needs to be in range of the enemies at the beginning of the foe's turn, which increases her chance of getting swarmed/killed. Her passive Desperation helps with that, but a bulky green unit can ignore the speed boost and one shot her anyway.
She's still a good melee unit, but my best friend and I are using Sharena 5* (him) and Corrin [F] the dragon user (me). Sharena's 5* skills reduce ATK by a significant amount to make her unintentionally tanky (good to have as a melee unit), while my Corrin is a debuffer that reduces the resistance of units she fights against in range (good to have when you have a kickass mage/tome user, since it helps guarantees kills even against units of the stronger color).
Since you have both of those heroes in your arsenal, you can choose to alternatively level them up and have a team with far more bulk/defenses (Sharena + Merric = Mage with high HP/ATK boost and tanky thanks to Sharena's damage reduction), or a more aggressive playstyle (Corrin[F] + Merric = enemies are more susceptible to dying by magic).
But ultimately it's up to you - your team will work fine for now, but hopefully you'll get more orbs and better units on the their list.
Make sure you do a full summon when you're starting off. Consuming 20 orbs for 5 units in one session > Consuming 25 orbs for 5 units for five separate sessions. Get your castle up to max level if possible as well.
Don't do any skill inheritence, hero sacrifices, or merging until you get AT LEAST 80 units in your hero box, or unless you have like, 8 copies of the same hero. [I wouldn't do it then still because you haven't encountered IV calculations yet, which is for later, so save your heroes just in case].Any other questions? ^^;