I would say pretty much never. I did briefly entertain the possibility for taking out scarab, since I was having trouble getting my team to work using a dedicated champ wearing destroy gear, but very quickly found a way to get the team to work without resorting to that.

With that said, I should note that I'm not quite at the point of being able to do FW21 on most factions; I'm in the process of building up my teams for that, and I do anticipate needing to swap gear around to clear that fight. Still, I don't think it'll happen often, since I'll want those same teams to be able to farm FW20, and I don't think the gear differential between 20 and 21 is significant.

I am taking the slow, patient approach as well.  I think on some of the FW21 stages though a couple gear swaps will be needed.  Like I can 1 star FW21 but the boss ignores defense so I think making gorgorab high HP instead of high DEF will work (I don't maybe he will get 1 shot either way and I just need to manual the boss better).  Or I could just wait for good rng... lol

I've done it a few times on some DT floors, yes, for instance temporarily putting a resistance banner on Arbiter that reduced her speed and hence her usability in arena. It's not ideal and I generally prefer to just use different champs that have higher resistance (or accuracy) instead, but in some cases it's the easiest way to go. 

I'm somewhat behind your progress - only 1 FW finished and currently stuck on level 40 hard DT. 

My clanmate was stuck at 40, built and booked Brogni...now through 70 I think lol

Better than me - I'm stuck at DT10. Can't seem to dish out enough damage quickly, and don't have enough healing to prevent him from getting those big extra turn hits when HP drops below 50% :/

Good to hear, I'm also building and booking Brogni... he's looking excellent indeed. Doubt I'll get it done before the DT reset, but that's okay, next time. 

I think Artifact swapping/sharing is required for Faction Wars.

(At least in the beginning when you are trying to get all the stars for Lydia.)

- 13 Faction Crypts

- 5 Heros per Faction Crypt Team

You are looking at using a minimum of 65 Hero's in all.

- 6 Gear Pieces per Hero

You are looking at using a minimum of 390 Gear Pieces in all.

- 1 to 2 Mil Silver per Max Upgrade

You are looking at using a minmum of 390 Mil Silver to 780 Mil Silver

This Silver amount isn't factoring in Accessories.

This Silver amount isn't factoring in any 6th hero's you might need to build.

What often happens is you will have a core of 5 heros which help you go from Stage 1 to 20.

Than you might need a 6th hero which has a very specific skill set to help you deal with Final Stage.

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Lets say for agrument sake, You decided to gear swap.

Lets say you have 1 Full Suit - 4 Piece Shield set + 2 Piece Immortal set

This 1 Full Suit will cost you 6 mil to 12 mil Silver.

Ideally, you would want this suit to be used on an HP hero's in each Faction Crypts.

Your plan is to upgrade it to max the first time.

Than swap it from Faction hero to Faction hero.

The Upfront Cost to max gear will be around 6 mil to 12 mil Silver.

The Gear Cost removal will be 600k if all 6 items are 6 star gear (Lets Assume it is)

Swapping this 1 set of gear 13 different times = 7.8 mil Silver.

You are looking at a total Silver Expense of 13.8 mil to 19.8 mil Silver.

It is a massive difference.

It is a massive time savor.

Keep in mind, This is for 1 hero.

You will most likely do the same process for 4 other heros.

It will cost you a total of around 100 mil Silver vs. 780 mil Silver.

Sharing and managing some artifacts between champs was normal to me when I was finishing FW. Nowadays I try to keep my builds fixed unless I'm regearing for improvement. Only exception to this though was when I had to regear some champs to finish the last DT boss, but after that I just put their previous artifacts back. 

Tbh even if we get free artifact swaps I'd still try to keep my regears/swaps to a minimum (thus keeping headaches to a minimum too I guess lmao)