Best way to utilise skullshield: skullshield hero points 20-40 , double strike (if you can) , Push the Limit to increase OH% like with the hero points , maybe even skilled mythic if it affects offhand primary skills . That's for generally using skullshield. That high hit frequency and high OH% DMG is very potent and the hits for crushing blow especially.
A likely combo can be a aftermath wizard like the charged inferno to utilise stack Debuff and high voltage on fire enemies, a wandering wizard or summoner wizard and basically any wizard. Although another way to get skullshield without the actual offhand is by the Reclaim Talent. This is great for bypassing certain restrictions on making a build and if you cast a lot , the skullshield will come. Also the necrotic talent to increase OH by 50%.
Default is 300% OH per bolt in skullshield. With skullshield 20, it's 600%. With skullshield 40 , it's 900% OH.
Push the Limit increases by 50% OH (if it's MH, it affects MH also) and necrotic talent also.
900%×1.5×1.5= 2025% OH per bolt. 3 bolts at once = 2025%×3= 6075% OH . But skullshield at 40 hits many times a second so it's brutally devastating, especially for bosses. Its basically a nuke!
At skullshield 20, it's 50% increased hit frequency . At 40, it's 100% increased hit frequency or 2x the hits it normally can. That's on top of increased DMG. Combined with double strike for another ×2 the hits (25% chance to do so is actually big when your skullshield rapidly attacks).
But if you had 100% Push the Limit , Necrotic 40 and hero points skullshield 40 : 900%×2×2= 3600% OH per bolt.
3600×3= 10800% OH combined. It can be done in less than 1 second and many times.
Even if you don't directly focus on the enemy with the skullshield attacks, it can be very good for Frozen explosions, Poison build, crushing flames + explosive build , arcane + orbit build , fire+ bleed , shock+ bleed , etc.
Do definitely put over 100% weaken so you deal ×2 DMG as I confirmed that 100% weaken essentially is dealing ×2 DMG through rigorous testing. How? 90% weaken + 30% epic affix. Try with weaken and then try with ignore resist and see the DMG difference. For me , ignore resist was 6M DMG on test build and 12M DMG on test build with 100% weaken. It can make a major difference in the billions also.
In PvP , skullshield in its own is very good in any build + Battlemage. Damage reduction by high AR and some resists . Fauns gift for HP and resists. Satyrs spirit for increased AR% but lower HP means you need to likely use something like alchemy + permafrost Regen or druidic bonus potentially. It can even work in Discordance Orb wizard or aftermath wizard in PvP. Reclaim also useful in PvP as a talent.
PvP Oh DMG is 300% but 1x Push the Limit is 18% instead of 50%. Although you'd probably use 2 for good measure. Also assuming you added +3 Skullshield at least. Even at 300% per bolt it's very good.
But I'm calculating what it would be for just 1 PTL (push the limit) and +3 Skullshield.
300%×1.18×1.15= 407.1% OH. 407.1%×3= 1221.3% OH combined and a few times a second.
There's also reclaim talent and necrotic. With necrotic 4, it would be +10% increased OH . That's 407.1%×1.10= 447.81% OH easily. Although with 2× 18% PTL , 7 Necrotic if you added +3 extra and +3 Skullshield, it would become 821.1% per bolt in PvP! On top of that, 30% damage Reduction!
821.1%×3= 2463.3% OH combined and few times a second.
Timewarp can also be an option for skullshield if it's a projectile since that increases projectile DMG by 50% . Cosmic power (5) buffs this by 62.5% so it ends up being much more.
Push the Limit for almost every build. MH% or OH%.
One thing to be noted , APS won't affect Skullshield. Would be ridiculously OP if it did. I did test it before so I know.