1. This matriarch just shot 2 venom attacks in a single turn that did 40 damage and wiped out my entire party.

2. The phase spiders get their venom attack every single round. If you actually follow the rules upon which the game is based, there is an entry in the monster tables for ranged attacks like this that says "recharge." It's in there to prevent these creatures from getting their absurd special ranged attacks every single round.

Here's an example: https://roll20.net/compendium/dnd5e/Giant%20Spider#content

These encounters are totally absurd and it's not the mark of a good game that you have to cheat and use foreknowledge to win every encounter. Please stick to the rules and create creature encounters that aren't ridiculous.