I don't know much about the champion, but looking at his kit I feel like he would counter the current Juggernauts. His q would steal some AD from 
's passive, his W and E would keep away the likes of 
and 
away from your team and his R can steal a ton of defensive stats.
Eh. In theory, he would seem to be a good pick, but he's actually quite average. Besides for the Trundle support fad, he's been unpopular and nowhere near a desired pick.
If you learn him, you should be able to find out the matchups he counters best, but I couldn't tell you myself.
Ever since my garens been on perma ban in ranked ive taken trundle as replacement because he literally shits on most toplaners post 6. Pre 6 youre weaker squishier and can very likely get killed if you engage to hard but with your ult you destroy. Only thing you have to watch out for is who you fight. Hes dtill melee so typical melee counters counter him. Im still working on items but bork randuins sterak warmogs and trinity are good picks. Hes a duelist essentially and in teamfights youre the anti frontline. Hes good basically. Only thing to really practice is timing your w and e and who to target r with. Q just spam. Also focus is essential. If you focus the wrong champ youre gonna get fucked in a teamfight but thats true for nearly all champs.
Trundle counters Skarner, but Skarner is still pretty jerky.
He's not great at ganking, but he is a scary presence in teamfights and pushing back lane from what I've seen personally.
He is excellent against juggernauts because he counters everything about them. He steals their damage, their durability, and their mobility (because unlike many fighter champs, juggernauts only have move speed to get around). Trundle is also an excellent laner (something that can make a juggernaut feel pretty awful since they usually rely on winning lane) while scaling into an exceptional duelist/splitpusher/frontline/teamfighter because of his many steroids and excellent self synergy. He is also a master of using his opponents stats against them, so even if he is behind, he can level the playing field by buffing himself while draining away what makes a Juggernaut so strong.
Trundle is and always has been an "anti-tank" but more specifically "anti-bruiser", and juggernauts are an even more ideal target for him to shut down.
Try a build that looks like this and you'll see how disgustingly good he is against them very clearly: 
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and then just work in whatever boots you want (usually 
) you become incredibly speedy, deal a metric ton of damage, kill towers in seconds, and are un-duelable, all while having a huge pile of health and resists.
