Just shooting out some ideas:
-lots of bards (give them Warhorns and lingering song so they have to spend only one turn on music) and if you use the books marshals from the miniature handbook and Crusader and Warblades from the tome of battle using devoted spirit and white raven maneuvers.
-If you've got cliffs or mountains they can set up avalanches.
-Hobgoblin "adventurer groups", that are formed more like modern day elite striketeams. At this level they can even "scry-buff-teleport".
-Likewise scry-buff-teleport should be a long known tactic to them. Have most leaders either be or be protected by a caster with both detect scrying and anticipate teleport.
-There's a teamwork benefit in the PHB2 that needs only lots of 1st level archers and one ad best mid level leader, that, if I remember correctly, gives every archer +1 per other member shoting in the group. Scary




.
-At this level the hobgoblins are bound to keep some devils around.
-Warbeasts. Remember hobgoblins have no Cha penalty and are likely to have bards, so them convincing low int (6-8) beasts to work with them isn't unlikely. I'm personally a big fan of Behir in greatplate "tanks".
-If you have a brutish female character in the group, one hobgoblin that challenged her, got beaten and lived to tell the tale might fall in love with this inadvertly ideal of hobgoblinish "beauty" and have him resolve to get a better fighter so he can win her as his wife in a second duel. Or have him ask after he has beaten her in the initial fight. Or have a female hobgoblin ask this a male character and the mariage is the only way to get save passage for the group...
-Use unit tactics. Hobgoblins don't work in hordes, they build armies.
-Retreat when necessary and gather intelligence. A martial society like the Hobgoblin one knows how important it is to know your enemy. Use divinations.
-Remember that hobgoblins are a martial society of humanoids that have no average stat lower than humkans. One would think there's plenty of higher level guys among these tough constantly warring people.
-More to throw a curveball than making them dangerous, have them act clever in war, but honorable. The scry-buff-telekill group may first call out to them to prepare themselfes for battle instead of directly attacking.
Have the PC's come upon huge warcamps and instead of letting them slaughter endless warrior 1 hobgobs, have the champions of the army challenge them and let them pass when those champions loose. Have them heed the rules of law. If they loose but fought valiantly have the hobgoblins take their surrender, maybe even adoption into the clan.
Have human, dwarfen oder even elven warriors that where former captives that have adopted the brutal but honorable way of hobgoblins.
Have them infuriated if the PC's act dishonorable. Imagine the face on your players face when they get rid of some prisoners the fast way and later get captured and that obviously evil clad-in-cured-elvenskin warlord gives a speech to a gathered lynchmob how the prisoners won't be slain to show how "the way of the hobgoblins is morally superior to that of the soft and dishonorable scum".
Have a old dieing hobgoblin veteran ask the PC's to take his head and carry it as trophy to honor his worth as warrior. As bonus have him old and beaten, but alive and have him ask it before he attacks one last time-to fall in valiant battle with a worthy foe.
-There are most likely many different clans with very warlike politics between them. Navigating these politics may actually be the true challenge here.