As the title says, what can I do against a mid DenyPlank? Having faced several it only a small handful of times, I can't find a way to effectively counter it. The ability to auto push back to his own tower, the denial of exp, and the potential of a jungler places me in a woeful situation.

I'm often a player of Karthus. Without the exp to hit 6 at a decent my team loses some decent support from my ult. I have tried hitting the wraith and wolf camps when I can but find it counter productive to our jungler or dangerous because of their jungler and my lack of exp. It turns the game into a genuinely unfun amount of time desperately trying to keep myself worthwhile to my team without feeding.

Can anyone offer tips against DenyPlank? Particularly if I am playing Karthus, or just any general help would be most appreciated. Thanks.

Well, Gangplank beats Karthus mid, but everyone beats Karthus mid basically.

The general advice is to push his tower hard.

Reminds me of when I sololaned against an AP denyplank on TT...that was an awful match

Ward both sides, push hard, and wreck his tower. Poke at him with Q when the opportunity arises.

Pushing harder is generally one way to deal with it. Also why go for your wraiths? Maybe ward the bush and steal HIS wraiths? If you can do it safely and carefully that seems the better option.

As for doing it on Karthus specificallly. If he's under his tower you can keep harass up with Q while getting a few hits in on the tower.

Of course the threat of a jungle gank is pretty potent and you should take steps to deal with that.

A further tip? An early Chain Vest with intention to build an Hourglass may be worth consideration. Early Arm really cuts down on the damage you take from Gangplank's harass. Given that most GP's rely on crit...I think arm double dips vs it.

I know if I'm mid vs Karthus I'll rush a Hexdrinker on say Caitlyn as an example.

I don't play Karthus but I generally harass as much as possible against Denyplank, forcing him to waste his mana healing/harassing back with parley. If they carry clarity or teleport instead of the standard offensive or escape spells though, I doubt that this would work. Other than that, there isn't much to do but suck it up and buy wards or try and switch with your other solo.

Oh another tip. Since GP can't really push a lane quickly and a denyplank is likely to keep denying when your not in sight....
It's relatively safe for your tower to go gank a side lane. At level 4 you can do some damage for your sides whom given the presence of GP in mid, are a good bit more important for that match then you. An early kill will also let you catch up to GP. Just be careful not to steal all your sides creeps on them.

Another tip. To set up a safe exit proceed back away from the creep camps once toward your tower so he thinks your just playing careful from a gank. It's hard to see if you blue pilled or went elsewhere or what.

GP isn't very effective early game. He can't do much damage and all his mana is dedicated to raising minions and last hitting. You should be able to harass the **** out of him as Karthus. If you kill him even once you're upset his deny game completely. If you can make him spend mana counter harassing you, he now won't have the mana to keep raising minions, and you win as well.

Don't waste your time with the chain vest listed above, GP has enough armor pen to eat through it anyway, and you're not gonna build an hourglass as your first item. I start with a meki and two health pots, and first chance I get to go back I buy back up to five (and build a Tear). Your virtually unlimited mana and healing potential compared to his extremely mana starved one by comparison should give you the leverage to stop him in his tracks.

tear is all of 600 mana if you start with meki/saph, philo stones are 600 per for gp (assuming he starts with two faerie charms)

I think the best answer vs denyplank is to nuke the wave, then back up. If you nuke the wave fast enough (easily done past like level 3) it will hard reset each wave at the turret, so you can start back close to the middle of the lane again.