Hm.
Well, you won't have to worry about the death effect unless you have 10HD less than his caster level... Which is unlikely, IMO, or the challenge could prove to be simply too great for you to handle...
As for other ways of protecting yourselves from the automatic effects of Balsphemy... (note that if Blasphemy is a spell-like ability or a supernatural ability, some of those options won't work)
- Silence cast near (just behind) the BBEG might help.
- If Blasphemy is a spell or a spell-like ability, wouldn't dispelling / counterspelling it be an option?
- Death Ward against the death effects
- Freedom of Movement against the Paralysis
- Restoration to heal the weakness
- Spell Immunity
- Dispel Evil
- Spell Resistance
- A ready action to get the hell out of there when he starts casting Blasphemy (or a ready action to move out of the area when the cleric or wizard (who also readied an action) shouts "Blasphemy! Run away!".
- A ready action to pummel the guy when he starts casting the spell
- (not sure about this) a ready action to cast a wall spell between the BBEG and yourself
- Holy Word

- Repulsion (keep the BBEG 40' away)
- Project image (for wizard)
- Bardic Countersong ability?
- Anti-magic Field
Good luck!
AR