Magnus

Countered by

Weak Against

Assassins - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Adagio - can snowball Magnus in the early game

Blackfeather - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Idris - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Ishtar - can snowball Magnus in the early game

Malene - can snowball Magnus in the early game

Koshka - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Reza - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Taka - As Magnus is squishy, immobile, and not very strong in 1V1, assassins perform quite well into him

Yates - Early game pressure

Crucible - Block Magnus's B in team fights will greatly lower his damage output, as he will not be able to combo with A

Counters

Strong Against

Heroes that can zone people into one area
Heroes that can keep him safe
Heroes that can engage for him

Ardan

Churnwalker

Grace

Lorelai

Lyra

Phinn

Yates

Warhawk

This section is a work in progress

aristotle (10 Silver, NA)

Early Game magnus will have a lot of trouble csing, but with enough practice you’ll lh every minion!  He’s very powerful at level two, esp with the blue buff. Be very careful not to be stunned or caught out (you’re super squishy!). Start with two cp crystals, then build into halcyon chargers first item. I prefer spellfire as first damage item, then depending on game will build into shatter glass or frostbite. Clockwork third item (but build a relfex before that). Buy a shard and then build into aegis.
You need to be able to keep up with cs and gold to make magnus work. Keep checking ghe scoreboard! Tag along with ur support to rotate top to snag a kill or two 

He has amazing poke potential, especially at level 8! Always save ur ult until you’re sure you can kill at-least one enemy with it.
Lyra works really good with him, or lorelai. Yates also can work with his B stun (it’s got a long cooldown tho so be sure it’s up b4 ur support pulls)

Biggest tip is to practice and watch gameplay of him! He’s super fun and cool

mathweev (10 Bronze, EU)

In teamfights for mid/lategame, try to preserve your positioning and skill usages (use mainly probing attacks with the purpose of keeping Arcane Mark on your enemies along with various item effects (spellfire, frostburn, clockwork to refresh your abilities like a badass) untill the enemy team commits to the fight. I sometimes deliberately let my opponents use up their ultimates and part of their skill combos (especially agility based ones) as they're trying to take down my support or allies in order to counterattack. If done right, your enemies will have spent their momentum by the time you either attack for real (or ambush them) from the backline as they enter your range while being focused on your allies, thus leaving you with a relatively easy team wipe. I can only speak from personal experience in that it's incredibly satisfying and overpowered. On the flip side, positioning really is key for Magnus, and no amount of defensive items can make up for his vulnerability when caught in the open.