This is how to beat the Dragon as a Barbarian below level 50 (personally I was level 42) and is also how to perform one of the Barbarian's long/endless (with some luck) damage combo chains making the game much easier.
Firstly, stats wise just so you know that you're in and around the right area. I had 407 HP, a natural 84 defence, +8/7/6/6 (I think) on red/yellow/blue/green gems and 8 damage on skulls from leveling and equipment. (There is a necklace that adds +5 to all stats when upgraded to epic and Scalemail items have a lot of defence). Ideally you want Daemonic items as these add to skulls damage, +2 at epic level (I only had one of these, the fight can be made much easier with several). You want a one handed sword (personally mine was about 20 to use and 30 damage but I never used it once in the fight) and an Epic large healing potion (costing around 10 action for 19 healing points) just as backup in case your combo fails or you have a 'dead' turn you don't want to waste. When a combo ends you should have masses of action points if needed.
For skills pick Barbaric Roar, Tribal Mark/Brutality, Enrage, Luck and Warcry. When the battle starts try and get 3 green as soon as possible if you don't have +3 starting green from stats. Cast luck and now when you match yellow gems you get an extra turn (hugely useful for keeping the combo going). After this set about getting yellow and blue mana, you only need 4 red to start the chain really so you can pretty much ignore this and purple is entirely useless to you unless it's as a 4 grouping for an extra turn.
The Combo: Cast Enrage followed by Tribal Mark when there are at least 16 reds on the board (this will get you a further +4 skulls damage bonus). It can take a few castings of enrage but you should have little problem keeping the chain going. Alternatively, if you went with Brutality (this is by far the better choice if you have a high base skull damage) simply collect a few extra blue and cast it (you want about 15 after casting brutality before starting the combo chain). After this collect mana again if needs be and just cast Enrage followed by Warcry. Simply repeat over and over. (Make sure you have 25+ red mana so casting Warcry does not end your turn, shouldn't be a problem after any enrage) Make sure the dragon's defence is always down with Barbaric Roar (It only costs 3 yellow and doesn't end your turn). In between casts of Enrage match up all non-red 4-5 gems as well as any yellows (as these don't end your turn) and of course any groupings of 4-5 skulls.
Enrage very rarely covers +5 skulls so once you get a few up with Warcry there is little space and you should get a 4 in a row 95% of the time. Also when the +5 skulls explode they should keep you topped up enough in blue mana that you need not worry about it. Always use Warcry if you have 9 or more yellow mana (red should always be maxxed unless you've just cast warcry). If you have 8 or less then Use enrage again to try and get a good board to boost it up.
The only way this combo can end really is if you get unlucky with an enrage that fails to connect 4 reds or (more likely) you run out of yellow/blue after a few bad turns. If this happens (it probably will a few times in the long battle) try and match up the red 4 in a rows hoping for a lucky cascade or simply take the biggest matching damage group and return to step one or even use your healing potion/sword if there is nothing better available.
In many cases you can hit the dragon for 90+ damage on one good cascade alone during the combo and you can even stop him using his poison breath if you get this going quickly enough. (I just missed out on this) Personally I kept him locked in for around 700 damage before my combo failed and only needed a second chain to finish him off, leaving me with 242 HP left when he died after 34 turns.
Skills - Barbaric Roar, Tribal Mark/Brutality, Enrage, Luck and Warcry. Cast Luck followed by Tribal Mark or Brutality to get your skull damage up then repeatedly cast Enrage followed by Warcry matching any non-red 4/5-in-a-rows as well as any yellow gems for a long/endless combo.