+dmg% on hero points page usually means increase your MH% or OH% DMG . For eg, my meteor has 750% MH DMG or weapon DMG as some people call it or the game calls it.
If I have hero points 20 in Meteor which adds +100% weapon damage, it actually means 750%×2= 1500% MH meteor DMG . There are other parts to hero points also.
For the OH% where the weapon is offhand, if I have shatter at 500% OH or 500% weapon damage and I are 20 hero points into shatter, it doubles into 1000% OH because it multiplied by +100% even though it looks like it adds +100% damage. Multiplying by 100% means ×2 or (1+100%).
Manashield and Timewarp doesn’t actually deal DMG but manashield does deal reflect DMG . Although reflect DMG % isn’t based off any element but it’s a reflect attack that happens when you hit. It usually does x% MH DMG to enemies so 250% reflect damage is 250% MH dealt . Although mana shield reflect DMG is a different case where it reflects 200% OH when hit. With hero points, it basically adds 50% extra DMG to reflect at 20 hero points when it adds +50% reflect dmg% .
As far as the actual damage . At 20 hero points, it would suggest to add 100% more damage or double the original OH% since manashield is offhand % but manashield doesn’t do damage, only offhand reflect damage. It probably affects the offhand reflect or something else such as maybe adding damage to any skill when the shield is up.
Timewarp hero points probably increase the Timewarp potency on affecting projectile DMG by 50% to make it probably double but I can’t be certain. Timewarp is a buff and Debuff skill that can affect any character that casts projectiles but slow down any enemy attacks, usually projectiles but you can make it affect melee attackers with a wizard talent. However Timewarp itself doesn’t do DMG so it can confuse some about the damage added at Timewarp hero points.
Shieldwall also has reflect on OH% aka offhand reflect % instead of Mainhand reflect %. Reflect DMG legend affix is MH% normally.
Reflect doesn’t sound all that powerful unless you have the right build but it can be increased by Mirrored Chestplate , certain talents , Redirect, hero points, etc.