So, let's say we've got a character who can learn both Wizard's and Cleric's spells up to 9th spell level. The said character knows that he will face something that deals massive damage (and, by additional rules, it makes him roll Fortitude so he just does not die right away) and wants to find a way NOT to roll that Fort regardless of amounts of dices that will be thrown at him (so he, you know, dies if his HP are less than -10), and preferably at least for several minutes. What spells can he use to do that exactly? 

The rules of massive damage are:

> 
> If you ever sustain a single attack deals 50 points of damage or more
>  and it doesn’t kill you outright, you must make a DC 15 Fortitude
>  save. If this saving throw fails, you die regardless of your current
>  hit points. If you take 50 points of damage or more from multiple
>  attacks, no one of which dealt 50 or more points of damage itself, the
>  massive damage rule does not apply. 
> 
> 
> 

The amount of HP cap steps by 10 for every size category different from Medium (not that it really matters in the question), and as far as I understand, if one wants to cancel the Fort roll, he has to be immune to crits at all (through, I may be mistaken about it). 

This is not question about invulnerability/invincibility, so those won't do the trick. Damage Reduction won't do it as well. No magic items or even a slight failure chance