Ice Fragment skill card can stack +100% damage to counters, and this bonus is doubled when the attack doesnt break shields. You can't always stack counterattack damage to 3000%, but when you can...
For new players, these upgrades will make your life a lot easier. Once you get around 1000-1500 upgrade currency you can reset your points, fully unlock each tree, then reset and pick the big upgrades like the select character copy and epiphany rerolls.
If you remove 2 of the epiphany cards you can guarantee 2 copies on a card. With enough rare event chance and picking as many ? event areas after the 2nd zone halfway marker, its very likely you get the 3rd copy.
Also check out the resources tab on the right and you can check Sproots Spreadsheets for general character builds and the E7 wiki for chaos encounter info and a deck builder with equipment library.
Luke has 'Activate a card' engraving then 'Move all persona cards', so you can't loop for free. However, if you reduce them to 0 you can play a support's persona which can stack up a lot of damage for that turn.