It's the same as any other kind of tanking. Only in this case Plarium have gone out of their way to make tanking easier by making the main criterion the number of stars. And (I just discovered this yesterday) apparently you don't even need a full squad. Just 4 mechs will do!
As an example for league 3, in theory you can be in the 12-14 range with 3 4-star mechs and one 2 star mech. With up to 20 energy capacity each for the 4-star mechs, It's almost a guaranteed win for such a player.
Also, if such players happen to lose two rounds, they can't play a 5th round. Any time one of my games went to round 5 and the oppo didn't show up I used to think they were rage quitting because their tanking didn't work, but now I realize they didn't even have a 5th mech to play round 5.
I don't use mods (my own choice.. I never liked the concept) so I can't play league 1 anyway, and league 2 has way too many players with 3 purple star mechs in their squad (each still counts as 6 stars which is ridiculous). Defeating such players with a balanced 5-mech hangar is difficult to say the least. As a result hangar showdown for me became too stress-inducing over time, instead of being challenging and fun.
A couple of days ago I picked some lower tier mechs and tried league 3 instead. I had an advantage over low inventory power players (because of the weapons I can put on the mechs), but such players were the exception. Like you've mentioned, I mostly faced end game mechs.
Just like in tournaments, instead of stopping tanking, Plarium's lack of action is encouraging everyone else to tank as well. I've more or less stopped playing hangar showdown, except when I am in the mood to try specifically for those 3 fortune keys :-).