Gunships, bombers, air transports with cargo, vanguards should be able to take it......i dunno if it's pure shellers then even well microed dox or bomb bots should suffice, t2 bots of all kinds should also be really good.
Air units are good. Shellers can work. Holkins/Catapult works. Sniper bots work in theory, but in actual application shellers are too accurate and have enough AOE to take out blobs of attacking bots. My favorite ground tactic is to use Vanguards, Shellers, and Stingers. Vanguards stop all ground attackers and soak up damage, Shellers destroy everything, and Stingers take out all incoming air attacks. It's a deadly unit combo. I'm not aware of anything that counters that aside from more units or a nuke.
Levellers are actually really decent against enemy T2. And T2 assault bots have really, really good dps, and can eat most units alive!
The difficulty is getting Slammers close enough. Normally they're practically completely destroyed before they get close enough. :-/
Tiz difficult. Personally id like to split the sheller into a anti-building artillery, and anti-blob artillery to keep the one artillery from being good against an area and buildings.
I just want the Sheller to be less accurate like the pelter. I'd also love it if the Sheller was rather accurate while standing still but a lot less accurate while moving.
That could be cool. Im still a bit uneasy about that much aoe with that much damage in a mobile unit.
Yeah. I also don't like how the current sheller is better than the Pelter, is mobile, and even costs less.
Dox can take out unsupported Shellers. Microed GIL-Es can do it, but it's really risky. Gunships slaughter them. Catapults spank them, though I'm not sure that's a cost effective counter. Myself, I just drive my own Sheller/Leveller/Vanguard army at them.
for me, this is where a well-placed holkins can come into play. catas are excellent structure denial with pin-point accuracy. but holkins has good ol' AoE.
Just with buildings you pretty much can't, and that is the way it should be. But with units as others have said air units work perfect if it is only sellers. If it is sellers with air support and or tuns of AA bots then you need a mixture of your own units to counter his/her mixture.