Question on IVs

W-Tharja: +Def -Spd
I Use the same during TT and is part for my current ( and still on process) Arena Squad, in my experience work fine, kills few blue units (basically just the ones that not surpass his Def/Res a lot), enough to kill a couple of B-Lyn and Green Units

Normally At the end the shot that kill is Ignis/glacies so the +Atk is not Totally necessary

For cover the +4Def/Res from TT you can use a ally support also with fortify Def/Res for C-slot and Fortify Res/Def Seal.

But overall i think Both can work fine.

Before you merge the copies, look if "Vengeful Fighter 3" can work with other of your armor units, maybe W-Lissa

You generally want to go for +Spd if you want to increase your doubling potential. And Katarina already has such an abysmal Def stat that dumping it further will not change your match-ups very much. All this means is that if you do end up with a -Atk Katarina, as long as she's +Spd, she is still very much relevant with the right build.

Any -Spd/+something (except +HP) is great on Winter Tharja. I had a +Res/-Spd and she's still quite strong and has been my support character in the tempest trials. The IVs will pretty much dictate what special you want to use on her (i.e. Glacies or Ignis). Then there's the stat boost from weapon refine to consider. I made my Winter Tharja have a well-rounded defense with even Def/Res stats. While you want to avoid -Atk, it's not her Atk that makes her shine but her Vengeful Fighter skill. This is why having a 4-charge special like Glacies or Ignis is very good on her and she will always retaliate with a follow-up (essentially being able to do a double attack) on every enemy initiation if she meets the 50% HP or more requirement. So any unit that doubles her is only making her more dangerous the closer she gets to using her special.

Winter Lissa works best as an initiator because of her Bold Fighter skill. But having a good amount of speed will also prevent the defending unit from doubling you back. Lissa will always double if she attacks but if the enemy survives the 2nd attack, they only need a speed of 31 to make a 2nd attack on Lissa in their counterattack. That's why a more ideal build for her is having a brave weapon and since you have a +Atk Lissa, that's an even better weapon for her because the enemy could die without even having the chance to counterattack whereas Slaying Axe may provide that opportunity. Slaying Axe only lowers the cooldown requirement by one but you still require the actions to charge it. Every hit is considered 1 charge as well as taking a hit, including the double attack of a Brave weapon so in a sense you get 2 charges out of that one. Because of Bold Fighter, you will charge that special in no time and Brave Axe makes her get there even faster.