here's what i did in my DL game to make the hobgoblins a lot more dangerous, though my pc's were lev 15 and had arcane and divine magic galore. I designed a special piece of equipment for their scouting patrols. It was a belt with eight chains that looped off the belt to be held by everyone but the central caster the patrols consisted of one level 9-11 favored soul (or mystic from the dl book, i can't remember which i used, even cleric would work), one level 7-9 sorceror, and seven level 5 fighters. the favored soul would stand centrally with the sorceror directly behind him. the fighters would fill out the rest of the 3 square by three square area on a battle mat leaving the favored soul surrounded. the sorceror would cast invisibility sphere to set up the ambush and the favord soul would cast silence. they would wait until they could get at least one fighter on each party member with no more then one attack of opportunity and charge attack with all the fighters having the casters provide cover fire and healing as often as they could. the fighters would attack clear melee threats in two's and casters singly unless the party was small enough for the patrol to double up on arcane casters (i had a lot of players who could only make it half the time). grapple the casters, tie up the melee guys with numbers, little bit of healing, litle bit of arcane damage, nearly wiped the party a couple times even when it was at full force.
the party had been stripped of a good ammount of their equipment before starting out by an army they had the bad fortune of losing to in a war between the solamnics and the knights of neraka so their ac's were fairly low.
also, toss in a few dragonspawn templates to the adventure, acid or fire spitting goblinoids tend to freak out players. nearly wiped the party at level 10 with a pack of 20 goblins with that template.
also, grapple, grapple, grapple, use the assisting character rules to the monster's advantage. one grappling level 5 fighter hobgoblin with 6 others who also hit on their touch attack assisting is a tough roll for any level 12 character to come out on top of. nobody likes getting grappled. even if they don't end up getting hurt, it adds tension.
And i second all that red hand of doom stuff, just ran it in my iron heroes meets forgotten realms game (yes i know the two are almost the antithesis of each other, and no it doesn't really work out all that well but the players are having fun not needing a cleric or wizard in the forgotten realms) and that module makes for some damn tough hobgoblin based enemies, you could get it and just filch the hobgoblin leaders from it.
