I'll PUG just about anything in Warframe, except Spy missions and Cetus/Vallis bounties. Spy because most people are retarded and I can do all three twice as fast as a group normally manages to do them (or it's a sortie and practically guaranteed one of the three will fail), bounties because of assholes and host migrations.
Limbo for Coildrive though? I saw on reddit that Ivara can cloak it but they said that the rift doesn't protect it for whatever reason. Probably also a bug if it's true, but unlike the Ivara thing which I'm sure will be "fixed" tomorrow, a bug that isn't in player favor can take week(s) to get attention.
I think Gara is going to end up being the best choice for the highest tier bounties. She's nigh-invincible herself, the wall can fully protect one thing provided you build her right and are good at creating max height walls, and the only real nuisances are excavation bounty steps. Might also have to spend time at the start of a top tier bounty series building up a huge splinter storm, and then be extremely careful to never lose it, but at least you don't have to leave Vallis to keep bounties rolling.