The Dragon has no minions for his hp-burn and can not be provoked, so at round 3 of the fight he doesn't bring much to your team.

That said, he may have some utility fighting the waves in round 1 and 2. You need at least 200 accuracy on him to make his provoke hit, so accuracy/perception sets are a solid choice. 3 sets would give you 120 acc just from set bonus and make it much easier to achieve 200 with substats and great hall bonus, without need for an accuracy chest.

His damage is def based, so atk as a stat has no value at all. I used def% for gloves and chestplate and ignored his crit completely. The damage from his A1 is rather low. The A2 could hit really hard with 100% c.rate and high c.damage, but I think making an AoE hitter out of Drex is not the way to go, especially as that would limit his value vs. Spider, where the small Spiderlings shall burn and for that reason not die from his hits. And completely ignoring crit makes it easy to find artifacts for him that you wouldn't use on other champs.

tl, dr:

3x perception set, at least 200 acc total, chest and gloves def%, boots speed, no crit rate or crit damage needed

Bump for the EDIT.  

In addition to the stats mentioned above you need resistance to avoid that he gets to many poisons.

I run Drex in a Spider 20 team, where he has weak affinity and therefore gets attacked by the Spiderlings and they get hp burn from his passive. I disabled his A2 on team settings.

I tried him in stage 25, but with strong affinity he works quite bad in this dungeon. Without provoke he doesn't get attacked by the Spiderlings in stage 25. So as an exeption to the rule of bringing strong affinity champs, for Drex vs. Spider it's the other way round. If you use him in stage 25 (or other non-spirit affinity stages), you need his provoke to get attacked. Make sure his crit rate and damage are low, so he doesn't kill the Spiderlings by accident. They are living torches that burn the big spider to death and shall not die.

For his passive debuff placement he needs accuracy. Dungeon level x10 is a good rule of thumb for any debuffer in normal PvE dungeons. In the end it's the same gear I mentioned above + resistance from banner/substats.

I run my Drexthar with 241 resistance at Spider 20, and that is not enough to resist the poison from the Spiderlings all the time. I still have to improve his gear. A resistance banner is a must have. With 241 res he gets a poison in maybe one third of all attacks and dies in most runs (Scyl as reviver saves the day), I guess you need at least 300 res to avoid most poisons. 

Have you tried him with 

Lifesteal and Speed (Gloves and Chest)...

I have seen multiply using that with succes.

(I haven`t tried thst myself, but he is one of my next champs to be reorganised)