How would you fix the weakest codex?

[Fueldrop]

Wyches should be Ws 5, bloodbrides Ws 6 at base and have an effect like Bladestorm on their melee weapons (Let's call it "Knife-fighters, and it makes sense as wyches are supposed to be able to take down enemies regardless of size)
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If you're going to go that route I'd suggest dropping their ballistic skill by one from current, representing them focusing on one aspect of combat at the expense of another (I'm kinda hesitant about having troops at WS 5 for any army, but it does very much fit with the fluff for them.)

Really, I'd be inclined to consider giving Bloodbrides the Precision Strikes and Rampage special rules, and giving witch derivative squad leaders +1 on their dodge save while in a challenge, to represent them being able to focus on dodging a single foe's attacks rather than having to watch a whole bundle of attackers.

[Djbz]

If you're going to go that route I'd suggest dropping their ballistic skill by one from current, representing them focusing on one aspect of combat at the expense of another (I'm kinda hesitant about having troops at WS 5 for any army, but it does very much fit with the fluff for them.)

Really, I'd be inclined to consider giving Bloodbrides the Precision Strikes and Rampage special rules, and giving witch derivative squad leaders +1 on their dodge save while in a challenge, to represent them being able to focus on dodging a single foe's attacks rather than having to watch a whole bundle of attackers.

Oh,yeah one lower Bs wouldn't be too bad (It is something I thought of, just forgot to write it down), after all the Succubus has a lower Bs but higher ws than an Archon so the wyches having the same differences when compared to Kabalite warriors would make sense.

[Reldane]

Let's call it "Knife-fighters

instead lets just call it rending, yeah so it also has a minor effect against vehicles, but Witches are strength 3 anyway so it barely makes any difference

[Djbz]

instead lets just call it rending, yeah so it also has a minor effect against vehicles, but Witches are strength 3 anyway so it barely makes any difference

They're gladiators they should be killing people, they have no business stabbing vehicles.

[Reldane]

They're gladiators they should be killing people, they have no business stabbing vehicles.

I wouldn't have thought that Genesteelers or Razorwing flocks had much cause for killing vehicles either, but Universal special rules are meant to be universal. rending on low strength attacks isn't going to tear a vehicle apart.

[Grand Master Raziel]

House rules to buff individual armies are always going to be suspect. People buffing their own armies are rarely going to be objective enough about it to get it right.

A better way to go would be to work out some sort of handicap system with your regular playing group. I haven't felt the need to implement it yet, but I came up with the following system.

1: Keep track of your record vs all your particular opponents. If they play more than one army, differentiate between games played with different armies.

2: If you lose 2 games in a row vs a particular opponent (assuming he plays the same army both times), the next time you play that opponent (and that army), add 100pts to your list. For every subsequent game you lose to that opponent/army, add an additional 100pts to your list.

3: When you win a game vs that particular opponent/army, you lose 100pts off your bonus, and the count resets - you have to lose 2 games in a row to start receiving bonus points again.

So, let's say I'm playing my Chaos Marines vs Carol's Sisters of Battle. If I lose twice to Carol, I add +100pts to my list. So, if we were playing 1500pts, I'd actually be using 1600pts to her 1500. If I lose again, then next time we play I get 1700pts to Carol's 1500. If I then win, I go back down to 1600pts, and I'd have to lose to Carol's SoBs twice in a row before I began getting bonus points again.

[Fueldrop]

I am disagreeing that Witches should be rending in any variation.

Where to Witches train? The arena. What do they fight there?

1) Other Witches. No need for rending there.
2) Slaves. While slaves can come from any race or faction, it seems unlikely to me that they'll be allowed to keep their power armour. The spectators want to see suffering, not witches beating up a tin can!
3) Exotic beasts. No need for rending there.

I'm seeing Witches fill the role of Horde-slayer/crowd control, so lots of poisoned attacks that hit often is kinda their thing. They're a troops choice, they don't need to be all things to all foes. Let the raiders, ravagers and trueborn deal with vehicles, the Witches are out for sport.

[Path Walker]

I wouldn't have thought that Genesteelers or Razorwing flocks had much cause for killing vehicles either, but Universal special rules are meant to be universal. rending on low strength attacks isn't going to tear a vehicle apart.

No such thing as Universal Special Rules now...

- - - Updated - - -

instead lets just call it rending, yeah so it also has a minor effect against vehicles, but Witches are strength 3 anyway so it barely makes any difference

Or why not ignore rending and have it like tons of other special rules do now

[Fueldrop]

Actually, I did have one other idea for a rule for Witches:
Spectators: Witches are performers and will fight all the harder when eyes are on them. If there's an unengaged friendly unit within 12" of a witch squad engaged in close combat, the witches gain the stubborn special rule.

[Archon Charybdis]

I am disagreeing that Witches should be rending in any variation.

Where to Witches train? The arena. What do they fight there?

Lelith Hesperax has AP2 all the time with a simple pair of daggers, it's hardly a stretch to say other less talented Wyches have a lesser ability. You could very easily represent it as their ability to find openings and weak spots in an opponent's defense and going for the chinks in the armor. That's literally what Bladestorm does in fluff for Craftworld Eldar--you could copy paste the rule and it would work mechanically and thematically. It's not the only possible fix that could work, but it's hardly a bad or unfluffy one.