Dragon's Dogma 2

An easy way to this is being an archer (or warfarer equipped with a bow).

Many guides will tell you to shoot the unmaking arrow at him. That instantly kills any enemy.
That's an option, but you only get up to two of those arrows per playthrough and using one here seems like a bit of a waste to me, you can save those for harder bosses.

Explosive arrows are easy to make and do the job too.

Just shoot explosives at the purple crystals on his body.
The first shot plants the explosive, shooting again with a normal arrow causes an explosion.

With a strong bow it just took me one explosive arrow per crystal to destroy it.

Start shooting the first crystals where you first come in shooting distance when he is still in the water. Then hurry on to the beach to welcome him there and destroy the remaining crystals. You'll easily have them all destroyed on the beach.

The Gigantus will still keep walking on his scripted route for a few minutes until a cutscene plays where he is killed near the camp, don't worry. The moment the crystals are gone is what counts, the game just needs him to reach a certain point to play its story cutscene so he's a dead man walking.

The trophy unlocks when the cutscene plays.