Sooo… if Ariel were to use Thunderwave on the guys in black, and Bardic Inspiration on Pacha the peasant. How would I do that?
Ooh! Here we go...
OK. So, Ariel would be able to do both at the same time because using
Thunderwave is an Action, and Bardic Inspiration is a Bonus Action. However, after she does so, battle will definitely start and everyone will have to roll initiative.
I must also add that Ariel will need to move up to the guy to use Thunderwave, because the spell isn't a projectile that Ariel can lob at the enemy. This spell is centered on Ariel and will do damage to everyone within 15 feet of her, and push every unsecured object away from her. So beware, this will effect the other customers at the diner too.
There are other spells that Ariel can use in this situation as well!
Vicious Mockery is an attack cantrip that Ariel can use. Should the foe fail the Wisdom Save against Ariel's spell DC, not only will they take a little bit of Psychic Damage, but they will also have Disadvantage on their next attack (this means that they will roll 2d20, and take the lower result. Advantage is the opposite, 2d20 and take the higher result).
Cantrips can be used at will and as many times as the caster wants.
1st level spells and higher will have a certain amount of spell slots as characters level up. This means that those spells can be used a certain amount of times per in game day. The used spell slots return after characters take a long rest (i.e. going to sleep). Right now, Ariel only has 2 spell slots to use for 1st level spells.
Ariel also has
Silvery Barbs. This is a spell that is done as a Reaction, instead of an Action or Bonus Action. In initiative, this would mean that Ariel can cast this spell when during someone else's turn. Ariel can use this to force the guy to reroll and take the lower result if he does something like attacking the peasant, and this allows her to give an ally of hers an advantage.
A note on Reactions: Once Ariel does use a Reaction for Silvery Barbs, she won't be able to do any other Reactions such as Attack of Opportunity.
I've linked the spells in question to this post. I hope this helps. ^^"