Buy Force of Nature if you getting absolutely lit up by AP. If you need a bit more magic resist and want to keep up your damage buy wits end. Thornmail is the ultimate AD carry and melee heavy counter. If you are already satisfied with your tanginess after Maw and they have some armor buy Last Whisper. All these situational items are possibilities to replace the thorn mail only. I do not recommend removing any other item. The only other items I can see that I might recommend but haven't tried out are Tiamat for the AoE as well as the health regen while adding some damage and bilge water cutlass for that nice life steal sustain. This last slot is what really seals the deal on you owning any champ that your playing against so you need to build it according to what the other team is purchasing as well as who is giving you and your team the most trouble.

Well another good summoner spell for trundle would be ghost to add to his movement speed when initiating. I just like heal because it makes it possible to solo dragon early and I'm able to gank a lane and turret dive to get the kill knowing I will make it out alive. Flash is always viable if you prefer also.

Other jungle trundle guides recommend AS Marks or a mix between AS and ArP. But for this build the focus will be on having a very strong late game or post-guardian angel. I'm not saying your early will be terrible, actually it should rock after lvl 3 with a red buff. Seriously you can successfully pull off lvl gsanks with red buff and your vamp scepter a lot. These runes make it not necessary to buy Last Whisper or any ArP item unless you want the damage and the team has armor items. The seals make your early game clear very easy and you take almost no damage. The Mresist is scaling to be better effective in team fights where its going to be needed.

Trundle for me is played in a way I don't know many others play. I focus on my first clear before anything unless a lvl 1 team fight occurs. You require blue buff the first clear but by the second time I find it is unnecessary by the time it has respawned and it will better benefit a mana hungry champ. Use your bite 24/7 when you have blue. Same with your W for that matter. When your blue has run out switch to only biting when the buff from it wears off and only use W to clear red buff, Drag, and lane covers. As well as ganks. Your pillar should only be used in ganks or when your covering a lane and someone is being pesky. Also its a great escape tool especially in jungle. As Trundle I tend to be covering lane a lot when my red buff is off CD. Ganking is frequent also and counter jungling is another thing. Just try to help out your teammates as much as possible once you have your ult. When team fights break out you may discover a couple things. A.) Your team wins or B.) Your team loses. If your losing just tell your team to keep on the defensive and go farm as quickly as possible for your guardian angel. Also try warding with your wriggles to find a enemy champ all alone. But I guarantee the guardian angel will change the tides of battle.