Your biggest challenge in getting Dragon20 down will be surviving, both the waves and the boss. You have no CC, which will make the waves challenging, and for the boss, you don't really have enough healing.

Apo is not a bad option, but he won't really be enough on his own likely. Still - my vote would be Apo and FB. Your D20 team, for now, will be Deacon, Apo, FB, Artak, and Ronda.

Why Should he be going for dragon 20 (sorry if that's a noob question ... I am a noob)?

Dragon 20 is generally advised as the 1st dungeon you should try to push and farm.  Mostly it's because speed and lifesteal gear drop there.  Speed is incredibly important everywhere and lifesteal is a solid set for early game clan boss.

It's also the easiest dungeon to push though Spider is much easier now with Artak being a login champion.  Ice Golem has a difficult boss with extremely niche sets and Fire Knight has tough mechanics and sets more focused on PvP.

Aiming to farm Dragon for speed and lifesteal gear is advice that seems no longer applicable for newer players - there's so much other content that will help progression better than just targeting one dungeons (ie, Missions and Challenges). 

Additionally, the availability of leech makes farming for lifesteal redundant - especially with Deacon as a starter promo code champ - plus the ease of getting perception gear from Faction Wars means you'll end up with plenty of fast options without having to farm dragon specifically for speed artifacts.

Dragon will be the first dungeon players are able to defeat at high levels simply because it's easier than other dungeons, but there's no need to focus specifically on farming it at first as there's so much useful content and events to cover early on that wil get you a ton of useful stuff.

Farming Dragon certainly still should be the focus. The sets are still quite useful, missions and challenges don't get you good gear, and gear drives advancement in the game.

And farming specifically Dragonn you can most quickly get to Stage 20, then 25, which means more quality 6 star gear. This is the way to start collecting good gear with which to advance in Clan Boss, the other dungeons, Doom Tower.... and even yes, even do those missions and challenges for the comparatively paltry rewards they give.

We certainly want to be farming Dragon 25 rather than FK 14 or IG 16.... for example.

I like Kram's team above of Deacon, Apothecary, FB, Artak, and Ronda. If you have issues with survivability, ranking up Djamarsa as a reviver could work well, but really that team should do well with decent gear.

you have some really nice champs but ur roster is very limited, u probably are better farming spider and dragon for now, as for starter cb team again ur really limited with what u have to work with but something such as a deacon/high khatun for some nice turn control and you might be able to do some decent dmg early game to it. lightsworn and aox are both fantastic champions by the way, id personally say both worth 6 star.

My advice is to crush your enemies, see them driven before you, and to hear the lamentaions of the women.