If Trump imposes tariffs on Canada, I hope Canada does not retaliate at all since that will only hurt us more.
If we absolutely insist on retaliating, then maybe we could impose an export tariff of $X.XX/kg or $X.XX/Kw as the least harmful form of retaliation if we need an emotional release to feel good about how we fought back. If we can keep a cool head on our shoulders though, then the best thing we could do is to not retaliate. If the US reduces imports through import tariffs, then that will force the US dollar up relative to other currencies. Then what? Will he just raise the tariffs even more? He could keep doing that until he tariffs the US into a corner once US products just become too expensive for the world to buy.
Unfortunately, I'm not confident that Canadians can keep a cool head in all of this.