All right, so I've been a die-hard Karchev player since I got my grubby hands on Apotheosis, and I've been getting some Mk. II games in with the big guy lately, and wanted to share some insights I've gotten lately from recent play, and pick other people's brains about tactics and possible army builds.
First, the major Mk. II core rule changes that benefit the good Kommander:
Trample: Trample has been really altered, and it really helps Karchev lists. We're all aware that Trample attacks no longer result in free strikes from the missed models, but I think what may be overlooked by some is that all effects that occur during the Trample are resolved when the model ends its Trample movement. So if a Berserker Tramples over some IFP, any Critical Knockdown will be resolved where it ends its movement, so it always gets the full movement, and always gets to make any possible Trample attacks. Trample used to require that you get at least 3" of movement (like a charge) in order to be able to make the Trample attacks and movement--now you can Trample models in base to base with the Trampling model.
Two-Hand Throws: MAT values went up, and now we can use them to make the ranged attack roll on Two-Hand throws. Throw damage is the result of a melee attack roll, so it can be boosted like any other, and that means Unearthly Rage pulls double duty on Two-Hand Throws.
Shake Effects: Karchev used to be hosed by Stationary and Knockdown, and now we have a counter for both. True, it costs a focus and only helps if you survive the turn when he suffers from it, but if anyone played Karchev against Stryker1 or Kreoss1, you know exactly how paralyzing that could be.
Crippled Systems: Since crippled weapon systems now only lose a damage die, Warjacks under Unearthly Rage attack just as well as they did when they were fully functioning, and on Khadoran 'jacks that means the ability to pile on the hurt even late into the game.
So how do these play into his new paradigm?
Trample:Well, first off, Trample gives us a way to clear infantry just using our jacks. Most players are still tempted to form their units into straight battle lines to maximize charge opportunities, which just makes them ripe to plow through. Assuming two rows of troops in base-to-base, it's possible to have a single warjack kill 6 in a single Trample attack without any crazy positioning, and I've found in my games that even against dispersed units 2-4 is reasonable, which is certainly better than any stock melee abilities on any but Beast-09 or the Devastator. With the increased MAT values on our warjacks, cutting through infantry screens is no longer the problem it used to be. On Karchev's feat turn the entire battlegroup can participate, and that means quite a few opportunites to kill small-based infantry.
Sidearms: practically a new spell. Knockdown used to mean that this was pretty much never worth casting, especially with Iron Curtain in play. Now the no-KD addition means you can run a slower list that waits for first contact with the enemy. You can fit up to 5 warjacks around Karchev's base without blocking his LOS, which with Sidearms brings him to DEF 17. If you circle them around his base, each one can be in base-to-base with him and also another warjack in the battlegroup, resulting in +2 DEF.
You can really maximize the DEF values by taking at least 2 Berserkers and 2 Kodiaks, so that the Berserkers are up in front of Karchev, and the Kodiaks are just behind them. The Kodiaks can Vent Steam to provide the Cloud Effect, and bring the Berserkers and Karchev up to DEF 15 and 19 respectively from ranged attacks. Combine that with Karchev's ability to camp Focus now and the bonus from the Arcantrik Turbine, you can easily get him to DEF 19, ARM 24 from Ranged attacks, which means only a Long Gunner CRA or something similar is going to be able to hit and damage the guy. With the old Iron Curtain + Tow combination, Karchev was extremely vulnerable to early game assassination, and with boosted DEF and ARM, that's no longer as much of an issue. Aside from hiding behind terrain, it's our best option on the approach.
Ground Zero: Better than it might at first appear, especially if you choose to plow through the opponent's army rather than trying to maneuver around it. With the changes to the charge rules, you can choose to cast Ground Zero before making any charge attacks and save your use of Karchev's axe for a model with a damage grid. At an automatic POW 13 within ~1.4" of Karchev, you can get up to 6 troops in B2B with the blast radius. On average you'll kill any single-wound Shield-Wall infantry trooper in the game. Those that aren't destroyed can get moved back d6". Since you choose the order in which they're moved, it's easy enough to make sure they get as much or as little distance you desire. The movement effect can open a hole in a line for Karchev to exploit, or to get Karchev out of melee so that he make a Fissure attack (or two with Reinholdt).
Unearthly Rage & Tow: Karchev now has the focus to cast Unearthly Rage, Tow, and use the Arcantrik Turbine in the same activation. The big plus here is that he can be using Sidearms on the approach and create a warjack wall, and then abruptly change tactics and make an assassination run. In a game about a week ago I did just that, and used the Turbine-Tow-Unearthly Rage combo to Trample 9" over blocking infantry to deliver Karchev and 2 Kodiaks to my opponent's warcaster. The Kodiaks popped out, made auto-boosted rolls against it, and assassinated easily.
Now, all of this isn't necessarily to say that playing Karchev with a ton of warjacks should be the norm, nor is it necessarily the most competitive way to play (especially against classic debuff casters), but I think there's greater flexibility now with using warjacks than there was previously, because we can layer enough DEF and ARM buffs on Karchev to make him survivable in a slower, more warjack-centric army list.