[Taneli]

I would build it as Affliction with the granted advantage to your maximum level, and then add a home-built limitation of being limited to target's Will/2 levels of the payload advantage. I would probably look at limitations like Accessibility, and would probably eyeball it to something like -10% to -20%

Do note that this kind of limitation is a bit annoying, as if your full power is below typical Will/2, or 5, it's not a limitation at all! Just about anyone could benefit from your full power. To get a fair assessment for the limitation, one would need to go into statistical analysis about the distribution of Will in the population.

But anyway, just eye-balling it, if you're able to grant 10 levels, and any one target only benefits of up to their Will/2 levels, I might be willing to accept something like -10%, and if your full power is something like 15+ levels (and thus requiring Will 30+ to fully benefit from!) I would be probably willing to accept something like -20% to -40%, as you're practically always limited to something like 1/2 or 1/3 of your full power.