OOC

[GM Marvel]

Acctually the robot vulnerability to critical hits is sort of unique

See the Robot entry in the d20pfsrd  here  https://www.d20pfsrd.com/bestiary/monste...cts/robot/

Vulnerable to Critical Hits: Whenever a robot takes extra damage from a critical hit, it must make a DC 15 Fortitude save to avoid being stunned for 1 round. If it makes a successful saving throw, it is staggered for 1 round. The robot remains immune to other sources of the stunned condition.

So it doesn't actually cause x1.5 damage but can be stunned by a critical hit instead

Also, constructs.. which robots are.. not objects., in pathfinder do not get Hardness, though they may get Damage reduction.  Only immobile objects get hardness. Usuallya construct made of a hard substance will have a suitable Damage Reduction ability

Well, this robot does have Hardness.  Hardness 10!  That will make it difficult to damage without a critical.  For those who don't know, hardness means you lower all damage by 10 points.  So if you can't do at least 11 damage, you will need a critical to damage the bot.  At least the group have two x3 crit weapons and one x4 crit weapon.

Thank you for finding answer.  I was reading the vulnerability and Hardness sections when I should have been looking at type.  But now that changes things...    Off to edit...