The Incredible Adventures of Van Helsing

You don't need to have a specific character build for either your character or Lady Katarina for this achievement.

All you need to do is to give Lady Katarina an armor that reflects damage. Obtain an armor that has more than 21 essence capacity and put 3 Ethereal Essences in it. Equip it on Lady Katarina and go to a spot where you slightly approach the boss and trigger the infinite spawn. Check the video linked for the exact spot. Whenever Bloated Gargantulas try to attack Lady Katarina, they'll be reflected 21 damage. Since it's above their 16-18 health points, they'll basically kill themselves. Just make sure to stay in a position where the Bloated Gargantulas will attack Lady Katarina (instead of you), and where Lady Katarina won't be able to attack Arachnid Leviathan on her own (a few meters down from the south egg spawn locations).

You only need to make sure that Lady Katarina will be able to recover from what small damage the spiders give her. As long as you spend her (and your) ability & skill points to ensure that, you can make her build how ever you want.

If you don't think you can make a build on your own:
It's recommended that you make a melee build for Lady Katarina with Life Regeneration (or Life Steal or Life per Hit) enhancements. Dump all ability points to Body. If you have the Blue Blood DLC, you can dump all skill points to her blue blood skills. primarily increasing Attack Speed. While the spiders will kill themselves, in my opinion it's better for her to kill some on her own too. If you have the Thaumaturge DLC, it's recommended that you make a thaumaturge character. His first skill, Energy Bolt, has a mastery called Spirit Bolts, which creates a bolt circling you every 5 seconds. It automatically attacks creatures if they're in your vicinity, so you can kill a spider every 5 seconds without doing anything actively while Lady Katarina and her armor kill the rest of them.

You might want to turn your video settings to the minimum possible for to account for worsening performance. It might allow you to stay there for a bit longer, but after about 6000 kills you should restart the area.

Edit: It goes without saying, but this is on Casual difficulty with a non-hardcore character.

Edit 2: Forgot that VH2 had Life Regen skill for Katarina, not VH1.