Well I'm having a lot more issues (this season in comparison to last) with this high damage meta that we are in. If you ask each morde main, he has a different style of playing. Mine was/is the tanky/run at you style of morde. I basically learned that it does't matter what you build on morde, its a matter of how you want to play him. Morde scales off of Hp, AP, AD, CDR, Movement Speed, Attack Speed, Armor and MR. He is unique in the fact that he is the only champ to scale off of of literally everything (except mana, don't build that shit).
If you talk to the Morde mains group chat, there is one guy playing Black Cleaver/duskblade Morde. There are a few guys running things like Ludens, Trinity Force, or etc...
The biggest thing is to learn how you want to play him.
If you want to nuke someone down, play the ludens/lieandris/sorc shoes route.
If you want to get in someones way, build things like Visage, and Thornmail. (I'll get back to this in my playstyle)
If you want to feel like an ADC get a Trinityforce and Stereks Guage.
From finding how you want to play (which I have done all 3 before, and more) its only a matter of knowing how fast you can kill the enemy vs how long you can live just like in any matchup.
So back to my play style. My play style involves a lot of micro-movements that optimizes a build to get in the way of the enemy as much as possible.
The easiest way to explain this is that for someone like morde who is slow to get all 3 q's off without the enemy just standing ontop of you, you have to micro walk with the enemy between each hit. Once you figure out how to micro-walk to get all 3 q's in there is only the basics of micro positioning for W, and E. W needs, NEEDS to hit two targets to get its full healing effect. For E all you have to know is its range and how it functions. If you have multiple enemies you just need to do a combination of micro walking and positioning to get the enemy where you can get max shielding/healing from the enemy champions. R is terrible to micro manage. The coding is awful on it. Honestly just spam the fuck out of the R button when you have a ghost, and hopefully it will listen. Controlling two champions at the same time is extremely difficult.
This is the easiest way to show you what something looks like. In the client this was recorded at HALF speed. So to watch it at normal speed, you must use the youtube 2x feature.
https://www.youtube.com/watch?v=cWFX2_yryp0
So the video starts with me INTENTIONALLY getting caught ontop of kogmaw. This did two things First, It popped Guardian, second, I was in position to get in the way of the enemy team. If you look, I had my W going on the kogmaw. I also popped it for a 400hp heal to deny the darius ulti reset. If you see dragon attacking, he has a short attention span, that was me literally spamming the fuck out of the R button. He still takes commands while you are cc'd. All of that focus on me, after the kog died, allowed for my team to do things, which again is what I decided I wanted my playstyle to be. My literal goal each game is to piss off the enemy team as much as possible. From there after my flash out, if you notice I pull dragon back to myself (using w to give it a little more MS) at the same time I cast a W to heal. Now here is the important part. Jungle monsters let you heal for the full amount. So I used Jungle monsters to get a full heal from W. I immediately start micro walking to get my Q's off. As soon as the 3rd Q is done I start walking out, only to micro walk back to where I get my E shield and another heal from the jungle creeps, all while trying to spam R on the kassadin. Again, that video was recorded at 1/2 speed. If you want to see it in full speed move youtube up the the 2x option XD.
So Long Story short, if you want advice from the better than me Mordes, they are going to say things like "Build more damage" and they are right, damage this season is key. I just don't like that style of play. I learn what I need to beat each champ individually, and then I build to counter my biggest threat, or help my ally. If you look through my match history I have some games where I'm support and I get a zekes herald and knights vow with banner of the command. Want to counter someone like Yi, go zhonyas and trinity force with sterkes gauge, as soon as he alphas 9/10 yis also use their e for bonus true damage, and they also used their ulti to get to you. So by time zhonyas finishes he basically no longer gets his buffs, now you are just free to auto him to death. You can round out that build with a phantom dancer for 12% damage reduction too.
The reason Spirit visage and Tmail are my two favorite items build wise is because I feel that they give the most value for their cost. Mordes shield Makes up for HP, so you don't necessarily need ALOT of hp to become decently tanky. If you properly micro-move to get a max W and E off, you can live for a very long time.
My playstyle often involves getting the enemy to over commit by making them think I'm squishier than I actually am. The biggest trick of all is to use W's bonus movement speed, and an ally or minion wave to trick them into melee range. Damage morde's tend to get around this either through ludens 10% MS or the MS bonus from Trinity force.
Tldr: Abuse the shield as much as possible to try and reduce enemy damage as much as possible. If they get close to you use micro-movements to reduce enemy damage/maximize shielding and healing. Learn what to build against what champions. Each item has its place in mordes kit. Its been awhile, but one of my tactics for awhile was to even build phantom dancer first item for the 12% damage reduction.
p.s. last season I would have considered myself high elo, diamond in two different queues, and plat in the other, however this season I wouldn't. I just haven't quite found my place in this high damage meta. I'm also glad you asked this question, because my playstyle is directly related to this thread. I got to a higher MMR last season through Micro-movements, and game knowledge. This season since there is a large number of champions that don't need to use mircro-moevements to be useful it makes it much more difficult (for me) to play anything that doesn't just instantly nuke down the enemy. I do strongly believe that if Riot wants to keep the game in this state its because higher damage lowers the skill requirement to play this game properly, which in turn makes it feel better for people that just want to instanuke the enemy. Its very similar to spawn camping in first person shooter games. It very much removes the need for skill.