Leviathan keeps losing

As ScoundrelRui01 said, run Scimitar. You should never lose running Fury-class, B-28, TIE Dagger with Mark VI and Scimitar as your only reinforcements and 6 dot mods on all crew members. Opening first turn is irrelevant. Hit Levis two specials, bring in Scimitar as first reinf., take out enemy B-28 with your Mark VI basic when it arrives as reinf., target enemy Dagger, hit full auto, win the battle. It's a painfully boring fight, but you will never lose a mirror match. Even with the odd AI enemy pulling in an early reinf. and/or the enemy taking out my Dagger, this strategy has failed exactly 0 times.