I've followed some redditor's suggestion and ran Spectral Speartip and her unique armour piece with an exotic name, that I can't spell properly. Shilebreaker had nice quirks like eldritch slayer and +melee crit.

Unparalleled Finese and Gallows Humour are excellent camping skills. Tracking Shot for bosses, Open Vein in position 1, Duelist advance for everything else. His grapeshot, which I've never used before, is great against annoying 'seeds of madness', and does significant amout of damage, when supercharged by quirks and Battle Ballad. I've intentionally let stress build up on him and got 'powerful' virtue, thanks to his 'Crimson Court' set. The fact that my riposte Highwayman was a force of nature with 3 hater/slayer and prismatic force quirk helped a lot. The End Game condition was +speed and crit on monsters. I got the 300 kills achievment with the Vestal-Jester-Highwayman-Shieldbreaker in the latest (and probably the last) 24839 build of the game with 'Crimson Court', 'Colour of Madness' and 'Shieldbreaker' DLC enabled (no mods). It's fairly common to be able to finish one with Intimidate or get a bleed on it that finishes it. For instances, those horses should be taken out as quickly as possible, before shuffling if possible.

That's the general idea, although I'll make exceptions and focus a back ranker sometimes. I suppose your party crushes rank 1 and 2 so quickly, that the backrow enemies are constantly within reach. I switch in Finale, Tracking Shot and Revenge for bosses. And Leper in front rank can hit the back rank with Intimidate if it's critically important to do so so I very occasionally do that and keep it on the bar for that purpose. HWM can also hit back rank with Pistol Shot if in position. In the end, I went with Shieldbreaker instead of Leper to reliably burst down rank 4. I wanted to try this as well, but I figured, that in this party only Vestal with judgement and Jester with Finale can attack position 4 (phase 1 of sleeper, foreman, virago, stress dealers, etc.). JES spams Battle Ballad, with the occasional Slice Off, Dirk Stab for unshuffling, and stress healing as necessary. VES spams group heals, single target heals, and attacks/stuns in roughly that order of frequency.

Darkest dungeon achievement guide