the problem you show is, you play a character that is specialized on critical hits (sneak attack) and ask how to make him better versus exactle the opposite?
Mmmhh... my first answer would be - become a Barbarian or Fighter.
But since this answer won't help you much - here are a few ideas:
Non-Critable Creatures normally only include golems, constructs & undead, right? Maybe even elementals... The only way to make him better (combat-wise) vs these foes is to increase your damage versus them. There aren't that many feats that will help you there (if you don't plan to take fighter class to at least 4th level). Power Attack might help als long as you fight low-AC opponents. Also, the combination of Power Attack & True Strike is not to be underastimated. You could try and change your tactics, by using Combat Expertise to wear them down with small hits (the idea of improved faint from those posted before me won't help you much, since those immune to critical hits are also immune to sneak attack... ).
Well that leaves specialised equipment against these creatures... there is a big bunch of specialized undead-killing equipment out there... with constructs & golems this can be a bit harder to find...
...but in the end - I think I would accept, that you are not a fighter and that you are at a loss when fighting non-critable creatures - you may still support a fighter-type when fighting them, but you will allways have trouble fighting them
