D&D 5E (2014) Your CR limit for an average 1st level party?

[Plaguescarred]

CR 2-3 depending on the monster's output and forgiving tactic used.

 

[Weiley31]

Didn't Icespire Peak send first level PCs against a manticore?

It did, but it also allowed you an "out" option to the fight by being able to convince it to move somewhere else instead IIRC.