I've recently spent a lot of time refining the Black Cat Voltron deck and trying to make it as streamlined as possible. So far I've been able to clear most scenarios on Heroic 1 - Kang Anachronauts is an absolute pain though. I've tried to go into plenty of detail on the various card choices and quantities:

The Allies

Key Allies: Iron Man and Ronin alongside Black Cat are essential to this deck. In order to function this deck NEEDS to draw 1 of these on your first turn EVERY game. Call for Aid is here for this purpose, and this purpose only. When you mulligan if you don't have one of these 6 cards then you discard your entire hand, NO EXCEPTIONS. Call for Aid really doesn't do much outside the first turn, but it is incredibly important for that first turn. Here is some maths to explain why:

If you only play Iron Man, Ronin and Black Cat then your probability of seeing one in your first 12 cards is 66.8% (40 card deck). By adding in 3 copies of Call for Aid this increases to 90.2%. (Note that this current build actually plays 42 cards so this number drops to 88.7%)

Supporting cast: Make the Call is here as a 2 of because if you discard Black Cat off of Call for Aid you can retrieve her. Having established why Call for Aid is essential to this deck, it is clear that this deck cannot play any other avenger characters as they would conflict. I chose to play only one other ally in Maria Hill because she is just a fantastic card. In the early game she can help with thwarting, blocking, and making your Make the Calls more likely to have a target. In the late game though you will eventually ideally have all 3 of your other allies in play so you won't be able to play her anymore.

The Upgrades

Inspired is easily the best upgrade to bring out early so if you do already have one of your 6 key starter cards turn 1 then you should keep this in your mulligan. It is the only upgrade which boosts thwart (Sky Cycle does too, but at the cost of additional consequential damage). It's good with all 3 allies.

Honorary Avenger is also really important to see quickly on Black Cat as she needs the avenger trait for Sky Cycle and Power Gloves. Notably this is the only upgrade which doesn't have a "max 1 per ally" restriction and it's also the only upgrade you can play on your identity. In the early turns this isn't brilliant to play onto Iron Man since all it really does is heal him 1, and you aren't getting a discount. It's good with the other 2 allies though.

Power Gloves is typically speaking a slightly worse version of Inspired because you can't play it straight onto Black Cat and it doesn't boost thwart. That being said, it's still a very important card because it's effectively just 3 more copies of Inspired and occasionally you can use it to pop toughs or play around guard. It's good with Iron Man, Ronin and only good with Black Cat once she's an avenger.

Sky Cycle isn't particularly great in the early game. On Iron Man and Ronin it can burn through their health a bit too quickly if you put it on them early, and it can't go on Black Cat at all until you have Honorary Avenger. With that in mind, if I were to cut any further upgrades to less than 3 copies, this would be the one. It's still really strong though later in the game, particularly on Black Cat.

Reinforced Suit is a really good upgrade on both Ronin (buffs and heals him) and on Iron Man (heals him for 0 cost). On Black Cat it typically isn't great as all it really does is give her the option of thwarting. For this reason I'm currently only playing 2 copies.

The Heal

Black Cat does not need to be healed very much since her big draw is that she doesn't take consequential damage. Iron Man and Ronin really like being healed though, especially once you equip them with sky cycle.

Since most of the time you only have 1 of these 2 allies out, it is much better that you play a heal card which heals 1 ally a lot. Outside of Reinforced Suit the only card in the game that heals more than 1 damage is First Aid which is why I'm playing the full 3 copies.

Originally I was playing a 40 card deck without any copies of United We Stand but I realised that in longer scenarios you don't have enough heal. This is because as the game drags on all of your Honorary Avengers and Reinforced Suits are stuck in play, so you can only heal with events. United We Stand felt like the best card to play as further heal in a late game scenario since it has the potential to heal both Ronin and Iron Man at once.

Cards I'm not playing

Genius, Energy, Strength: These cards are almost always staples. However, this deck just wants to be as consistent as possible in drawing it's key allies early, and then have hand after hand full of upgrades/heal to support them. All of the upgrades are 0-1 cost, and the allies are typically only played once each the whole game since the aim is for them not to die. It would be nice to have the resource cards in combination with some allies early, but it isn't worth putting them in your deck. Peter Parker, Iron Man and Web-Shooter ensure you can always afford to play what you want anyway.

Team Training: This card is in theory really strong in a deck that aims to bring out allies and have them never die. However, it has 2 key weaknesses that hold it back. Firstly, it is a support, not an event. This means that once you play it it's out of your deck, and can't be played again. United We Stand on the other hand will always be redrawn after a reshuffle. This is important because you don't want to play too many heal cards in your deck as they cause bricky hands in the early game. BUT in the lategame all you want to draw is heal. By including heal events instead of supports/upgrades, you ensure that your late game deck is very dense with heal once you've played a lot of your non events.

Lead from the Front: In theory this card is a decent finisher, but in practice I haven't felt much need to play a finisher in this deck. By the time you're set up Black Cat is swinging for 6 every turn, and Ronin is doing a ridiculous 10 damage. The thing holding you back from being able to finish the villain is moreso having enough health on your non Black Cat allies to make use of Sky Cycle than not having big stats.

Inspiring Presence/Get Ready: These cards do combo very well with Black Cat. However, as said before, additional readying on Iron Man/Ronin isn't typically important as a lot of the time the restriction is their health more than their ability to ready. The heal from Inspiring Presence is nice, but I think I would rather heal 2 allies with United We Stand. Black Cat synergy alone just isn't enough to play these cards, and one ready of her is only ever at max worth 3 damage which whilst strong, isn't game breaking enough to make up for the downsides.

Extra Allies: As said before, Call for Aid getting you to Ronin/Iron Man 100% of the time is far too important to play any other avenger allies. That being said, the deck definitely could play additional non avengers (e.g. Ironheart, Nick Fury, Mockingbird could all work in here to further synergise with Make the Call in the early game). The main reason I'm not playing them is to keep the deck streamlined and consistent. The other reason they aren't great is they aren't compatible with the fact that I'm playing 0 resource cards.

Conclusion

This deck is really fun to play, and it is strong too. It really struggles against ally removal cards though, so be careful against Anachronauts Kang!