It depends what at what stage of gearing this is. With list of items that are in the guide (cheap/easy to get), you have no business doing samael, it will be too weak.
Eth su axe is decent because of sheer dps of eth item. Proc will not do much unless you manage to stack decent mount of lightning pierce. And yea 15% is very low in comparison to old values.
I think bis weapon for the build is probably shaad rw now in eth labrys, it provides highest raw damage, good aoe proc, and defense bonus per rune makes up for that loss of few runes. Not that defense would matter in samael fight. But average damage over 800 can't be beat by craft. Maybe some crazy near perfect roll with correct pierce and bunch of deadly strike. But shaad is way easier to get in comparison, even eth base is porlly easier to buy/find.
Pretty sure minmaxed single target spec would work around kraken stance and pierce all elements runes. New ele weapons can roll +kraken stance, helm can, so you can hit ridiculously high base level. Then you pump nearly all points you can into stance tree that works as synergy for kraken. Really most basic glass canon, output as much damage as possible, while having enough health to stay alive. Such build is terrible to farm with but for certain bosses, especially ones that you can die vs, this is exactly what you want.