Alright so I main 
and I do pretty good with him. Except I always have a hard time dealing damage to tanks. It's not the magic resist that they are building that is the problem but I don't really have a way to take down all that health they are building.
What %hp damage item is good for ap users?
Kouga2 votesI mean there is the % HP damage AP item.
Xilixer1 votesIs there no additional items besides that one? I really want to shred some health.
Serevas1 votesNope that's it.
Kouga1 votesIf you really need to deal with tanks you can combine it with a 
. That will make your E and R slow which will increase the overall effectiveness of the HP burn as well as give you even more kiting by slowing them. If you can keep slowing and burning them it may help burn their health down. If they stack MR then go with a 
EndlessSorcerer1 votesVel'koz's ultimate actually has a slow, so it would only really help his W.
Spicy Rice1 votesi mean, mages aren't supposed to deal with tanks. Thats the job of the fighters/bruisers
EndlessSorcerer1 votes
Mages deal primarily burst damage and excel at bursting squishy targets. However, they are cooldown reliant and as such typically have lower DPS, which can make it difficult for them to kill tanks.
Vel'koz is actually one of the better mages for killing tanks due to his low cooldowns, synergy with Liandry's Torment, and kite-oriented playstyle.
A good build on Vel'koz (especially against tanks) in mid-game would be something like the following:
Afterwards, you would probably just build more ability power (i.e. Deathcap) to improve your damage further.
AthenasVendetta1 voteslets be honest... a 
.... that's a lot of damage. Now could it be you are not reaching that peak fast enough?
Critmaster Garen1 votesmages arent supposed to shred health or blow up tanks.
even when you cant take down a target, mages will still be relevant due to the utility in their kits. focus on killing the squishies, otherwise act as a second support.
Xilixer1 votesThx for the tips. I also want some opinions on my typical build when I'm doing good.
Here it is: 
