A better counter is Rotom FCO. It deals most of Trev's damage right back to it as a one card tech.
The problem with Rotom FCL is that it puts 2 damage counters back on Trev. An OHKO only happens in the case of a full bench with Skyfield. The bigger issue is that Rotom only removes 2 damage counters and not 3. Since Trev's first attack does 60 damage, it still 2HKOes a Rotom. Sure, you get another turn or two and can possibly kill one Trev/Trev break, but after that what happens? It also is very irrelevant in many other games, and a 1-of tech can be prized.
As for Zororark, why? The benefits of Zoroark is the ability and the attack. Retreating is not usually an issue in Night March. Retreat costs are low and the only non-attacker they can grab is Shaymin, which they usually KO. The reason you particularly want to put in Zoroark is for the attack. I have many problems with this.
1) What if they only bench 1 other pokemon? Many times Trev players are lucky enough to only have 2 Trevs down, meaning at max Zoroark is doing is 80 damage. While it is a 2HKO, Trev can easily take a KO right back because of bursting balloon and/or 30-60 damage from a silent fear.
2) It is hard to set up a stage 1 under item lock. Especially a 1-1 line. I remember playing the deck with milotic for testing when Trev break became big (yes I know about Puzzle, but no one knew which one was better at the time). What I learned is that Milotic as the basic had low hp and I barely got any return from it's ability as I generally used hex to get it out (in which feebas got KOed immediately). Zoroark is similar; the basic has 30 more hp, but still that's just two easy silent fear attacks and a KO. In addition, the turn you are likely to set it all up is the turn you hex, which means no stand in. Also, a 1-1 line is unpredictable under item lock as it can be prized or in the discard at any time.
3) There are better alternatives. I have already mentioned hex and lysandre. Those are your biggest helpers in the deck. Even if they get the turn 1 wally, all NM needs is one turn of items to basically win. You need 2 battle compressors and 1-2 Ultra Balls to get all the NM pokemon in the discard, which even after a supporter can work. There is also N. N was a great disrupt-er back in the day and is a good fit for NM. If they have 2 prizes left, a likely scenario for Trev, a turn where a lone Trev is KOed and an N can put them at a severe disadvantage. I am still toying with Hex and Mew as their synergy is a bit rough, but my line is currently 2 Hex 1 Lysandre. If you go first you are primed to win and within your 1st or second hand you are primed to have one of the 3 outs to item lock.
TL;DR: No Zoroark, yes Hex/Lysandre