A Plus Ultra finish is done by getting the final blow on an opponent using a Plus Ultra.
The Plus Ultra gauge is below your health and will show you how many charges you have.
Performing a PU with
+
will cost 1 charge.
Performing a PU with
+
will cost 2 charges.
Performing a PU with
+
+
will cost 3 charges (and requires having 2 sidekicks).
Simply wait until an enemy has low health and use a Plus Ultra to take them out.
Many story missions have a Plus Ultra finish as an optional objective to unlock everything on that level, so you'll be trying to do this a lot as it helps towards getting the items for other achievements