Trunda is great, definitely worth building, but my vote is to build Apoth out first. I think he'll help your dungeon farming team more, since otherwise you lack a healer.

I wouldn't bother with Spirithost. She has niche value in an old cookie-cutter speed nuke team, but that was a long time ago.

Skullcrusher will also be worth building, eventually, but I would start with Apoth and then Trunda.

Don't waste time with Tallia. Fenax will likely eventually be built, but not for a while. Vergis may be built, we'll have to wait and see. 

So, my thinking was that Apoth is lvl 50 ascended, fully booked, so he can contribute enough, but will it be that much better on 60? Well, I can put a banner on him, true

Bellower is great as a control champ for waves (especially in Stun set), build accuracy and speed on him. The key is to use the champs right. A good champ in a wrong role can be terrible. 

Trunda can be saved for later since you have Athel taking care of things for now. I would look at Apothecary because I think he'll help progress with your account the most. He's also a great champ for Fire Knight, where you can find Stun gear for your Bellower. 

He will be 60d eventually, so might as well. It's better to have a team of strong champs to get to Dragon 20 than a bunch of 50s. 

All right, then I go for fully upgrade Apothacery, and then gear up Bellower. Thank you for the help guys! Also I can use any other tips, so thanks in advance!

Trunda is a fantastic champ. Congratulations to this shard pull. She is probably the hardest AoE hitter in Raid.

You want a champ that can put decrease def on all enemies going before Trunda for a fast and functional team. I recommend to farm xp in campaign stage 9 for a while, where you can get Warmaiden as loot from campaign. Farm additional copies, you can use those copies instead of skill books to maximize her skills.

Warmaiden is not the best AoE decrease def champ in the game, but you don't have any so far and she can be got from the campaign as loot. For the better options you have to hope for a good shard pull. That's the reason why you should get Warmaiden first, so you allready have this ability in your team.

After getting Warmaiden go on farming in campaign stage 12.3. Make Apothecary and Trunda lvl 60 next. Bring your Warmaiden to 50.

A team of Apothecary, Warmaiden, Bellower, Trunda, Athel should be able to farm Dragon stage 20. Maybe reaching that stage, because of the affinity of the mentioned champs, is even harder than beating it. Stage 20 is worth doing for better gear. The drops you can get increase slightly from stage to stage, but from 19 to 20 is a big jump, because gear with 4* does not longer drop in stage 20. You get only 5* and 6* artifacts here. Dragon 20 is one of the most important stages in the game to obtain good gear.

To summarize: Get yourself an AoE decrease def champ. Make Apo and Trunda lvl 60. Farm Dragon dungeon.

The difference between 50 and 60 is certainly much more pronounced for damagers, you're correct. However, the thing is, Apoth is going to be the person responsible for making sure your team lives - and he can't really do that if he's dead.

save your shards for x2 summoning events, get a cb team up and running sooner rather than later, having good stats on your champions' gear is far more important than getting a set bonus, levelling gear below 5* up to 16 is a waste of silver (if you have to use lower ranked gear leave it at 12), and don't use epic or legendary champions as food.

now that i've been through my generic advice, was there anything specific you wanted to know about?

Okay, that was what I didn't know. Wether Bellower is enough for decrease def or not. So I have to have a 60% aoe version of dec. def. to be good. Thank you!

Okay, I see you now. Apoth needs to live

I got an answer above for the what to focus on next question, but these are good tips as well. Especially not to upgrade 4* artifacts totally. I made that mistake, so I'll focus on getting to dragon 20 not to worry about 4*-s. Thank you!

My personal view (some may disagree) is to focus on Dragon to get 5*-6* gear. 

Arena is important, but for now, I'd try to make it happen with the champs you have for Dragon. 

If you focus on arena early, you'll hit a gear wall. If you focus on Dragon, you may start slow in arena, but you will make it further as your gear gets better. I'd probably use Bellower as your Decrease Defense champ for now, only because you need to make sure your team survives. If you still don't have a 60% DecDef champ by the time you are ready for Arena, Warmaiden is a viable option.