As far I know dispel DC isn't related to save, it is about caster level so you should look into that. For example if caster lvl of the wizard who casted wall of fire is 7 your caster lvl check should past 18(11+7) to dispel it.
caster level of dispeller is 8; wizard's who cast wall spell is 10th level. would the dc be 10? Check would be d20 +8 and must be 10 or higher correct?