TL;DR: DR is not what you need, seriously.
Following DM manual, considering your PCs are lvl4 you need a CR 6 encounter to be a hard one. To achieve that you can make a goblin with 7 class levels or equivalent templates (one opponent CR 7)
Preferably you can make a multi-opponent encounter, for example:
- The goblin boss, with 6 rogue levels (CR 5)
- His bodyguard, a domesticated ogre (CR 3)
- 6 useless goblin meatbags (each CR 1/3, so it makes a CR 3)
total: CR 7 (the calculation is a bit complicated, you can find it here for example.)
Always prefer the multi-opponent encounter when you have the choice. Single opponents have en tendency to make less interesting fights (or frankly unbalanced ones).
So let's say you ended up with a CR5 boss with his minions. At this level you can't reasonably make your boss able to tank these 300 damages. The problem here is that your player is able to do that much, and RD will only reduce it by maybe 15, at maximum. That would make "only" 285 damages, definitely not a solution. Worse, that would mean that the other guy, the player who "plays normally", won't even be able to land a hit.
However if you want the encounter to be a challenge, you should better choose abilities which counter this very ability. Maybe it comes from sneak attacks: make the boss a barbarian or a rogue, or give him concealment. Maybe it comes from a ton of fire damages: give him fire immunity (or maybe just resistance) by finding the right template. There a weak point for every build and RD is not one for someone able to make 300 damages in one hit.