I disagree that a Balor will always or even often kill Jubilex.
In a straight up fight, Jubliex would have a fairly decent chance of defeating a Balor for the following reasons:
1) Jubilex is immune to the vorpal portion of the Balor's Vorpal weapon since he has ooze traits.
2) The Balor's weapons would quickly (one attack) be destroyed by Jubilex's Desolve ability and the sword would only average 5 points of damage on a successful hit anyway.
3) Jubilex is immune to the Balor's slam attacks.
4) Jubilex can spew Green Slime at the Balor every other round and have a 95% chance to hit. Granted, the fire of the Balor will kill the slime, but the slime will also do 1D6 points of Con damage to the Balor.
5) The Balor can throw quite a few different spells at Jubilex, but except for the few Reflex save ones, most of them do not have a real good chance of affecting him (due to SR and good Fort/Will saves).
6) A grapple would probably be Jubilex's best strategy. The Balor would do 11 average points of fire damage to Jubilex each round and zero points of bludgeoning damage. Jubilex would do 9+9 (two attacks) average points of slam and constrict bludgeoning damage and 2+2 average points of slam and constrict acid damage. And, Jubilex has more hit points. And, constriction grappling damage (depending on interpretation) is nonlethal, so the Balor's Death Throes ability would not come into play. Even if it did do lethal damage, Jubilex would often have more than 100 hit points remaining when the Balor dies (since he does twice as much damage per round in a grapple for many rounds).
If the Balor can summon another Balor, Jubilex could Teleport away.
The Balor is generally the better combatant, but Jubilex has abilities that make him the better (overall) grappler.
I suspect that a Balor would know this and would avoid the fight in the first place since he would not want to lose his weapons, just to get a few licks in before teleporting away.