In the Combo Master "Challanges" from the Dlc you have to hit the bosses 30 and 50 times without getting hit yourself, By Experimenting i Without a Doubt found the easiest way to Achieve this, However there is one pre requisit for this, that being you beat the Frozen Empress at least once with a Bayonett as the reward for this the Bayonett "Bloody Snow" is key as it´s Heavy attack fires 7-10 shots at once like a busrt fire Assault Rifle at the cost of 6 ichor. Since every "Bullet" is seen as its own "hit" you will have no trouble as long as you keep your distance and the boss is focused on your partner.
Though this strat is quite Ichor consuming, so i recommend a Bloodcode with a high Ichor stock. You should also equip passives that boost Your Ichor Stock like "Vow of Ichor" from the Isis Blood code or "Max Ichor Boost" from the Harmonia Bloodcode.
The Only 2 Abilties You really need to equip are "Valiant Heart" from the Survivor Bloodcode to make Dodging easier and "Blood Sacrifice" from the Hunter Bloodcode to regain Ichor mid Combat.

Example:
https://sethclydesdale.github.io/code-vein-builder/?build=IIBgPgogjFbgIuJcwGEDsaZoJxmFAKwrioBMAHGAGJRkklA