Against Viktor, it's reliant on you being 1) willing to back up to avoid giving him free klepto procs and 2) being willing to force an all in if he walks too far forward to get those klepto procs and knowing when to do either one. Taking ghost can help in this matchup.
Karma's only reliable poke also relies on her pushing the wave or positioning really strangely. If she shoves waves, she's more vulnerable to ganks, mobility or not. Additionally, you can take ghost into this matchup to help counteract the value of her mobility. Early boots can assist with this as well. Also remember to hold your e until the w snare if possible.
In both of these cases, your target is squishier than typical laners, so you will be able to afford putting off your damage in exchange for mobility and still be able to kill them. The key is itemizing in a way that enables to you get on top of them. Boots, phage, righteous glory, etc.
If you're against heimer, the early levels can be a pain. One trick you can do is to intentionally let a heimer tower hit you. If you've got second wind and dshield, you'll regen the damage, but minions will aggro to heimer. Level one, you can make less experienced heimer players take a lot of minion damage this way.
Aside from this, be willing to give up some cs and work to bait out the heimer e. If heimer's e is down, a significant portion of his burst damage and peel potential vanishes. Abuse it by abusing him. Itemization can be early mr first, since he's also squishy and base damage should be enough to kill him most of the time.
Renekton's abilities are each empowered on a 50 fury cast. Renekton gains 5 fury for each auto attack, gains fury for hitting enemies with non-empowered abilities, and also gains fury from auto-attacks using an empowered-w. A key thing to this matchup is being aware of his fury and playing around it carefully.
Additionally, he has a dash in his kit. Your q is signalled. Do not use your q if renekton still has e or you'll likely end up with him e'ing to dodge the blade, possibly to punish you by going into melee range to trade.
Renekton's typical trading pattern is to e through minions at you, auto w, q, e out. These short trades allow him to chunk you down slowly and abuse his own natural sustain in his q. Boneplating is good against this style of burst trading, take it.
Your cooldowns and renekton's are similar in length (aside from w, which favors you), but there's a difference between the two champions. He's completely reliant on his abilities. You aren't. Your passive means you win most extended trades. It is entirely possible for you to punish him if he extends poorly or even wastes a cooldown.
Most of your good fighting with him isn't going to be possible until around level 7 or 9. Renekton's designed to ruin your day before then. You can make this harder by putting off your damage. Early ninja tabi, prioritizing the ruby crystal over the longsword when building phage, etc. These things enable you to survive his damage output. And then you win the extended fight because, remember, you're darius. Your passive literally gives you a free bf sword and more.
As the game goes on, your ability to fight him should only get better, not worse.
Just remember, Darius isn't an early game champion. He's a mid-game champion. You don't win every laning phase. You can punish mistakes. But sometimes, the right choice is to farm it out to your powerspikes. Not every lane is about killing the other guy.
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