If you want to do it via melee combat, then you should train the dwarf up to legendary in as much stuff as possible (for stat-ups), and in the following specific skills:
* Wrestling (for dodging)
* Shield use (for blocking; a good shield-user is nearly invulnerable to strikes)
* Armor use (for speed in combat)
* Hammering (hammers don't get stuck in enemies, and therefore your dwarf is unlikely to lose their weapon)
Then you just kit him out with the best gear you can muster; masterwork adamantine is of course the ideal, but steel should be entirely adequate. If you can manage it, put the fight somewhere with limited line of sight so that enemy archers can't shoot him as easily.
But yeah, if your actual problem is "I have one dwarf, they have thirty orcs, what do I do?" the answer is a lever-based deathtrap. A single dwarf can construct a 100% foolproof deathtrap without ever getting into line of sight of the orcs if he has a pick and enough time. I recommend the "massive fall" trap type -- dig a deep pit, put a bridge over the top, and hook it up to a lever. Let orcs onto the bridge a few at a time using other bridges to control flow (don't use floodgates; they'll deconstruct if there's a unit on them when they close), then pull the lever. A deep enough fall (10 z-levels should do it) will kill the orcs, leaving their equipment behind to be safely retrieved.