When you're facing a strong solomid champion like Karthus, there are several things you can do. And there are several things that you should not do:
What to do against a solomid Karthus:
- Buy magic resistance and health potions, or a health item.
- Switch lanes with a more tanky champion (like Malphite), who can take the hits.
- Switch lanes with an anti-caster (like Kassadin), who may have an easier time against Karthus.
- Call for ganks often, even if your team does not have a jungler.
- Stay alive so that he does not get enough money for his core item! (i.e. Archangel's Staff).
- Let him push toward your tower so that your team can gank him or get tower hits on him.
- In ranked mode, choose a direct counter to Karthus (such as Soraka, who can heal everyone just enough to save her team from Karthus' global nuke).
- Wear him down with ranged attacks
- Later on, make sure everyone on your team has a Banshee's Veil
What not to do against a solomid Karthus:
- Do not stay in the lane against him as a squishy carry with no escapes (like Kogmaw).
- Do not "stick with your usual item build" and overlook the need for magic resistance.
- Do not stay in the lane with low health!
These are some great ways to lane against a strong early game champion. If you can shut down Karthus' early game advantage, you should easily destroy him mid and late game due to his lack of health.
Note: the picture of Karthus was taken from the following URL (courtesy of glodefunk): http://glodefunk.deviantart.com/art/LOL-Karthus-fsjal-164718697
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