To note, i assume you are gonna be doing delves solo. In case you are doing them in a group you would be better suited running a normal M+ build and playing like you would on a dungeon.
This is my recomended talent build for solo delves:
The main difficulty on higher level delves is gonna be keeping yourself alive long enough for things to die. Most of your damage wil come from Brann but in turn you will need to tank a lot of damage and also keep him healthy.
Your own damage will mostly come from max ranking fb. Because of chronof, afterimg and lfm just casting fb on packs will be almost all the damage you do and you will most likely spend the rest of your casts kiting or healing. You won't use disint at all and any extra gcds you want to spend on damage will be used on chronof.
Having only two friendly targets means there is no way to get good value out of ta. Not even noz can make up for it because ta is a somewhat long cast that feels awkward to fit when you're trying to be very mobile and react quickly to damage. Similar with stasis it can potentially be good if you need another button to heal yourself very quickly but we already have several of those and there is no real value on doing aoe healing.
During higher level delves or while undergeared melee hits or casts will do a big chunk of your health, potentially one or two shotting you without the use of defensives. Because quell has a 40s cooldown you need to be smart between what things you interrupt and what things you let go and heal up after, while also noting that Brann has its own interrupt that you can't really see the cooldown. Knowing which specific casts you can tank, which ones you can let go and dispell the effect after and which you have to interrupt everytime will require some practice and limit testing on your part. As you increase your gear or go to higher levels these limits will change but because delves are freely repeatable you can take as long as you need learning exactly how to play each type of mob.
as along with the slow from perchill be a vital part of your gameplay, letting you slow mobs while you keep them out of melee range and Brann kills them. It's not gonna be unusual for you to run all the way to the entrance of the delve while spamming as on a pack and Brann gets them down slowly.
One good way to handle difficult packs is pulline them with a single as and then running behind cover. This is a decent way to make Brann grab aggro of some of them while also avoiding casts. Dancing around pillars and walls can easily save your life from a variety of casts that would otherwise oneshot you.
Don't be afraid to use all of the enviroment and your kit, running away across gaps or hovering across mobs while spamming slows and healing yourself and Brann can very easily be the most successfull strategy.
To maximize the amount of damage Brann does and in turn reduce the amount you have to survive before things die check out the building Brann page. And if you want to see high level delves gameplay you can check some of the videos below or on my Youtube channel.