He's a very good champ and yes, you should build him - but whether he's a higher priority than others on your roster, or who you should replace with him, we obviously can't tell without seeing your roster and some view on your progress. 

Basically Royal Guard is good for high HP bosses, including some in the doom tower and in dungeon levels 15-20 or so - the ones where they didn't nerf enemy max HP damage. 

Oops forgot to add the roster; fixed it now

Royal Guard's value has gone down a bit since 21-25 caps MAX HP damage abilities.

Nonetheless, Spider 20 is worth farming for silver - it yields a higher silver:energy ratio than 25 does.

RG alone, though, won't do it. You'll need to build a team around him. So as to whether you should build him, it depends largely on where you are in the game. If you're at a point where you're trying to optimise your teams for faster clears, then he could help. But if you're struggling to advance, it's unlikely he'll get you any futther.

Royal Guard is definitely worth to build. Before dungeon 21~25 were added, he is in lost of end users' team for speed farming at dungeon 20s. Now his value went down a bit after dungeon 20 as Max HP damage is capped at 10%. For your current account, he is still worth for sure but I would say not at top priority yet. RG is not very easy to build and he needs lots of stats to shine. Focus on your campian progression first.

Royal Guard is still a top champ.

1. Stage 20 is, due to it's lesser energy requierement, still the best stage to do during a tournament. Maybe you don't get that much items you want to keep as in 21-25, but you get the best ratio for tournament points/energy. Royal Guard can deal heavy damage to the stage 20 boss, where his skill is not nerfed.

2. Royal Guard has an AoE max hp attack, makes him one of the best champs in dungeon stages 21-25. The waves are much, much stronger there as in stage 20, and the nerf to enemy max hp attacks only applies to the dungeon bosses. Royal Guard is your best man to deal with the waves.

Royal Guard is a very good champ however he only starts to shine later from lvl 15-20 dungeons onwards

at your current stage I'd probably prioritize other champs, like Armiger first