You should have specified this a bit more in the initial post....
What Bell 412 are you operating in this altitudes?
Classic, SP, HP, EP, EPI?
On high altitudes, the N1 and/or the ITT will reach the limit way before you will reach the torque limit-watch out for this.
(Not only applicable for the 412): Avoid rapid changes in the collective setting and/or fast controls in the cyclic or pedals, move controls smoothly.
Be aware of LTE-always land into wind.
Do not use the Autopilot (if installed) in ATT mode when operating in high altitude- The actuators will suffer, and will overcompensate.
Check your VNE and OAT-ALWAYS!
Keep your rate of D low, especially the higher you go....