IMO the dash is what makes Lancelot the only champion viable in the late game. The combination of big AP costs, low AP counts and lack of mobility makes champs absolutely non-competitive compared to other classes, because everything's already dead by the time they haul their fat asses to the front line. Meanwhile Lancelot also brings a shitton of utility with himself thanks to Inspire and Ice Spikes. A pretty great hero all around.
I'm playing this now and I've tried a variety of party compositions. I've had success with a shield wall composition, something like:
3 defenders, 1 champion: alternate Guard to give loads of extra armor so nothing can hurt you
2 defenders, 1 champion, 1 sage: equip some of the sage gear that gives +unbreakable armor to adjacent allies, with aura of protection it's easy to get +8 unbreakable extra armor every turn. Bonus if you find +damage to allies within X tiles gear.
Then have the champion scale massively with rage / gear that gives +damage per kill. I've been using the white knight and easily hitting 300+ damage on normal strikes.
Depending on your defenders / champion you also have some ranged options (the fire spell, Mordred's lightning) to take down archers rather than having to mop them up afterwards.
I can see this strategy working well if the game ever scales up rapidly (meaning lots of damage coming in or lots of health on enemies).
By far the most effective strategy I've found is to use 2 sages + 1 vanguard (assuming you're using Mordred, otherwise 3 sages would just make this stronger). I'm using Sir Tegyr, because I like him, even if someone like Boudicea is better due to her damage scaling (I checked the wiki and I don't see it listed, but in my game she has +10% damage per kill ability). Anyway, give the vanguard -1AP cost relic & movement AP on kill, pump them up with bless, inspire, etc. then 1-shot backstab every enemy on the map on the first turn. If you don't quite manage to kill everything, step back and stealth, then kill them next turn. I've stopped doing that because it made things too easy, but it's a fun option.
I haven't finished the game yet; just got to the start of Act IV, but I'm thoroughly enjoying it. Scratches the Battle Brothers itch I've been having.