I finally captured that lagiacrus :D It was quite difficult though. Not the fight itself, it was pretty easy surprisingly, but I couldn't spot it's limp too well, so I had to make him limp away 2 times before I heard the moan he was making, which confirmed it was cap time. I set up a shock trap underwater and friend royal ludroth came over and decided to use it >:c. Then I placed another shock trap, and lagi ran away. I tried to place another trap in the area he ran to, but only 1 shock trap at a time. Luckily, he limped over back into the other trap :D
Then I found out I could make thunderclap but I need dragonite ore, and I used all of my dragonite ore to make my ingot armor :\
And I got a rusty shard, and it made a really accomplishing sound when I got it, and i didn't see it in my inventory when I finished the quest. Can't that shard be used to make some weird rusty sword?