Quakers spawn at an uncommon rate, and they will not spawn if one is already alive.
Quakers are fast enemies which will wander towards their target. However, it cannot walk up non-sloped surfaces, which will cause it to jump and cause an earthquake upon landing. When the earthquake occurs, any entities near it are flung up at a high velocity. If any players are in the vicinity, they will be petrified for a short period of time.
The Quaker's affecting range is 50 tiles, so staying away at that distance and using ranged weapons will help with killing them.
Summoner minions work well too.
If you're using melee weapons and have the ability to dash/teleport, when the Quaker jumps, try and get away from it before it lands.