I finally captured Lagiacrus last night!
Really I have no idea why it took me so many attempts, it was really close too as I beat it into submission underwater, set a trap, it was inches away from it then it turned right around and retreated to the water cave in Flooded Forest.
Naturally I had a second trap but alas I could not use it for a full 2 or 3 minutes - however long a trap lasts when put down before expiring - so cue me trying desperately to keep Laggi busy by running/dodging out of the way while also avoiding the annoying Ludroth which kept getting in the way - and were eventually killed by Lagiacrus anyway - but finally the trap expired so I whacked down my trap there and then, was charged by the creature - knocked down to nearly absolute zero health - which was then caught in the trap so I scrambled to my feet, ran up to it and made him eat tranquiliser bombs!
Very happy to have that quest out of the way finally, I know I'm not advancing that quickly at the moment but I'm still playing every day, making a bit of progress each time, I'm gonna be trying to increase my HR rank by a bit over the next couple of days though hopefully.