I generally flip between 20 and 21, so I can probably help out a bit.
What you generally need to know is that you should have a core of three +10 units, and the last slot should be a bonus unit. The more merges you can get on your bonus unit the better, generally you want this to be the grail or 3-4* unit of the week.
Things that affect arena scoring:
Unit's base stat total
SP cost of equipped skills (the higher cost, the better scoring)
Duel skills and duo units status
Legendary units and other units in the team having the associated blessing, but you probably don't have to worry about this unless you want to stay in tier 21.
Thing you don't have to worry about:
The way arena scoring works, sadly you tend to be taking in suboptimal builds more often than not, my brave ike for example, has to use green infantry duel 4, despite DC being an obviously better choice.
Another thing to keep in mind is that your core team should have a good way of countering the current meta threats, for example my zihark here is built to kill fedelgard.