Kog’Maw is rather squishy in the early game. Try to abuse his weakness and look for fights to set him behind.
Post-6, Kog’Maw will deal lots of damage in a trade thanks to his W and his Ultimate R. Try to bait out his W before committing to the all-in.
Forcing Kog’Maw to overextend for farm is a good way of winning the matchup. As he will be overextended, this will allow your Jungler to gank your lane. This will also force him to use his Ultimate R or W to farm with- which you can capitalise on.
In team fights, try to lock Kog’Maw down as quickly as you can. This will reduce his ability to deal lots of damage with his W and auto-attacks.
At times, Kog’Maw will be left alone with his Support in the mid lane. You can abuse this by ambushing the two of them when the enemy is split up around the map. From there, you can take objectives in the mid lane.
Positioning is key for Kog’Maw. If he mispositions or walks too far forward to harass, make sure you capitalize on it and take him down.
Kog’Maw is weak in the early game. You can abuse this to get an early lead or blow his Summoner Spells.
At level 6, Kog’Maw will have an easier time poking and trading thanks to his Ultimate R. Take care when trading with him, and try not to let him burst you down.
Kog’Maw benefits from extended trades once he has a few points in his W. Do not go for extended trades with him unless you have an advantage.