The first two battles with Luca can be lost without any noticeable differences; the third battle is the only one that really matters. The only change is that (depending on whether you won or lost), Luca will have a different amount of hit points. Luca will have higher hit points if you lost the other battles, or Luca will have lower hit points if you won the battles.
General tactics concerning concerning Luca Blight have been covered in this thread: http://www.suikosource.com/phpBB3/viewtopic.php?t=507
Some of the good runes for this battle are as follows: Fire-sealing, Wall, Firefly (on someone with good DEF and MDEF), Earth/Mother Earth (protection mostly), Water/Flowing (healing), and Wind/Cyclone (healing and attack).
The balance rune can also come in handy if you decide to use any of the unites that unbalances one of its members (the Rival ATK (Valeria and Anita), comes to mind).
The Fire Emblem accessory can be invaluable since it provides 50% resistance to Luca Blight's fire attacks.
Although most of the statuses don't work on Luca, the Silence status does, which means no more fire attacks from him. Coupled with Humphrey with Firefly, this means that Luca will attack Humphrey every turn, dealing 3 damage each time he attacks. Poison can also deal pretty good damage to Luca, given time.
The combination spells are especially useful if you can pull them off.
Fire + Lightning = Blazing Camp
Lighening + Water = Thor
Water + Wind= Water Dragon
Wind + Earth = (forgot its name)
Earth + Fire = Scorched Earth
I can't seem to find the descriptions for the spell combinations, but I'm sure someone else will be kind enough to enlighten us.