I checked Healblade's calculations, and I think he made a couple mistakes that help a little. Lux will have 145 Speed after a B Agility, giving him a 41% hit chance Gorsia (runed). Also, if you get the upgrade from the guy in Melenam, Lux will have an extra 30 HP totaling 240. With runes and B Protect, Gorsia will hit him for 77-129, meaning Lux survives most 2 consecutive hits, and always survives if he guards one turn. At this point, Lejes could stretch the fight out 18 turns with 9 Recovery, 9 turns for 9 Potion 2. Lux, hitting for 99 max every 2 turns, will need at least 21 attacks to kill Gorsia.
To be honest, I just don't think we'd have the damage output at level 25.
Since I'm raining on your parade, I'm obligated to give you better news as well. I'll post a few lines of info, then explain.
With max power, at level 25, Lux has 133 Power and hits for 59-99.
With max power, at level 26, Lux has 138 Power and hits for 71-118.
With max power, at level 27, Lux has 143 Power and hits for 82-136.
With max power, at level 28, Lux has 148 Power and hits for 93-155.
If judicious use of runes, F Shield, M Waters 1 and 2, can get Gorsia fully runed, with Lux and Lejes potioned up, then we have around 27 turns to try to finish the job (plus another turn for each Potion 2). I think this is pretty much impossible at level 25, so let's check for higher levels.
At 26, Lux kills a runed Gorsia in an average of 22 attacks, down to 17 if we can get "lucky". At 27, this becomes an average of 19 attacks and a minimum of 15, which may be workable. At 28, this shouldn't be a problem.
If we can't manage Gorsia at 25, 1-2 extra levels would drastically improve our chances. By 28, this should be a blowout. But I think it's important to know how doable this is at 25-27 with the expected parameters.
I'm not really here. Pressing Up+Down too many times glitched your graphics. And audio. Yeah.