Corki just did a 1k magical critical to me

Yeup!Thats crit meta for you!And players will still blame Corki instead of the items!

Shiv probably in combo with his AA

You might have to link the game in question if you want anything more detailed. We can't see detailed info on your normal games otherwise.

Anyways, League of Legends tends to group damage of the same time if it occurs quickly enough. I think that the 1k magic damage would likely be a mixture of the following:

• Leona's passive would account for ~100-140 damage.
• Spellblade damage on Trinity Force would be split between physical and magic damage, accounting for another ~100 magic damage.
• Thunderlord's Decree could have proced on the same attack for 180-250 magic damage
• Corki may have hit you with an ability at the same time as the basic attack landed.

The other day I made a Crit Strike for 2.5k with Cait. I literally 1-shot a Rumble from full HP with just a single Basic Attack.

That's just the way the game works now: buy 3 Crit items and 4 AD items, then act surprised when you do unhealthy amounts of damage.