For Garen, you can try avoiding being marked as the villain because his ultimate passive gives him more damage, look it up at lolwikia, here.
Try using your slows after he uses his Q because it will cleanse all slows for him.
Stick near minions because as of right now, his E (spin) Judgement will deal an extra 33.3% of damage if he is only damaging one target with this ability.
You can also harass him so that his passive doesn't happen as often.
is typically the go to item for tank shredding champions like Garen and Darius. Because both typically stack health.
is an alternative if the champion you are playing is AP based.
For Darius, you can try avoiding his overall abilities. His passive bleed stacks will increase the damage of his ultimate. 5 is the maximum counter, in which his ultimate (true) damage will deal the most amount of damage.
Avoid his Q because it gives him great healing particularly when he is low in health. Look up ability details here.
Darius loses kill potential when his hook is on CD.
Most these are kind of obvious things to look for as well.
One tip that applies to all champions is to get a feel of how long their abilities are on CD, allows you gauge how long you have before they have that ability up again. A second tip that applies to all is to engage when their abilities are on cooldown.