I can't tell whether it's stuffed or the bonus effects, but the healing items trigger Leorilla.
Rotten Hardtack debuffs you, but that doesn't seem to matter. I had actually thought Seaweed had no effect on BtH, but it increases bonus by 10. Was it changed when Ruby started selling it?
Either way, despite having a maxed character with 70 nature/all resist, I can't beat the challenge using any of the strategies I know of, which include Patience Dragonlord, MSW, Paladin...
The challenge already needs a rebalance to fix Leorilla's immobility resistance (another attribute that's mauling my strategies), so I guess I can just wait.
I did the Weird Duo last night with Pally. Level 85, stats trained to 200 END, 200 LUK, and 20 WIS, equipment: BraveSirRobins Fearless Helm, Fragrant Oleander Cape, Grimey Ring, Leorilla Mane Belt IV, Cheesemonger's Gratitude, Elemental Unity Defender XV, Legion Bracer, and Elite Knife of the Sunken Rose III (with Blade of Awe shown). So that's 73 Nature resistance and -32 Health resistance, not the best possible but pretty good.
I used 5 health potions, rotten hardtack (which I don't think set off Leorilla, but I used it when he was watching himself for effects and may have missed it when he switched to watching me), 2 mana potions.
I used a combination of light of dawn, crusading strike, holy wrath, phoenix song, cor lucem, and normal attack against Bearginion depending on what was off cooldown and which element he was currently resisting, plus angelic blessing and greater healing when needed/available.
I think I used light of dawn a couple times on Leorilla when he was going to switch to watching me. I know I used stun 2 or 3 times (all successful - but that might just be the RNG making up for hating me when it comes to drops), no other buff/debuff skills, and no buff/debuffs at all once he was below 50% health. I also forgot to equip a pet once Bearginion was dead. And I finished the fight with close to full heath and both heal skills off cooldown.
So it's definitely still possible to do the Weird Duo with paladin. I'm not really interested in repeating it multiple times to see how much actual luck is needed in terms of misses, glancing blows, and crits.