I took a gander at your thread and have to say that, provided the splash is small, it is all much ado about nothing.
SR looks at caster level. AFAIK there is no monster in the game where you need max caster level plus max twinkage in order to overcome SR. That means missing a level or two is not going to have a significant impact provided the character has appropriate gear.
To lead with "multi-classed casters are better for using buffs" is just nonsense. In your own post you point out the problems with large splashes impacting duration and efficacy of buffs along with their increased chance of being dispelled.
A character with a small splash from some other class is scarcely less likely to overcome SR than a pure class character. A character with a large splash is more likely to face SP pool issues and provide weaker buffs that are more readily dispelled. So, your advice is wrong on both accounts.
Players with deep multiclassed casters should focus on spells that have no defense and leave buffs and resisted spells to the other casters. Players with minor splashes on their casters should continue on as if they were pure class.
Anyone can check out the spells for themself
here. There are many for which SR does not apply and against which even end bosses do not (normally) have protection.
Note: this issue of end fights is a bit of a red herring since there are very few where casters actually are the ones counted on for success.