But obviously not undefeatable.

We know that LTT made a quick surprise attack with 113 powerful male channelers and some ten thousand soldiers. Probably this army was very well trained, and very well equipped with *angreal and destructive weapons.

However, I think the main purpose of this force was to hide the (seven?) crucial channelers, whose task would just be the correct placing of the Sealing. It's e.g. possible that LTT ordered the other men to spread around SG not stay in a bulk, so that the DO couldn't strike at them all at once, or wouldn't know where to strike first. We also know that there are ter'angreal, which hide you from the Shadow, like the dull dagger. Maybe LTT and those other channelers, who placed the Sealing, had such ter'angreal to hide/protect them from the Shadow for a short time.

While we don't know, how long the Strike at SG actually took (maybe only minutes?), it's in any case said that no one of the soldiers survived and probably many channelers died as well before the DO could be Sealed. That seems pretty impressive. And it's said that "even Lews Therin himself, who intended to personally lead this huge raid, admitted that even with sucess, he expected few of the attackers to survive, perhaps none."

So, LTT considered it to be a suicide mission, which could possibly fail, likely because of the DO's vast powers at SG.

Edit: I just checked that besides LTT there were the sixty-eight survivors of the Hundred Companions. That means 45 channelers died as well.