One thing to remember is that a THMPR run in Em8-er probably won't be like Firefall. The enemies are going to be warriors, not the local fauna. Your average THMPR run in Em8-er is going to be like a Melding Thump in Firefall.
Additionally, when the Tsi-Hu attack us with their Kaiju, they're not limited by the number of players. As long as the servers can handle it, they can attack with whatever they have. While it's not forcing "skill" per se, it can force intelligent play or the usage of group tactics to drive the enemy back. Kaiju can also easily be designed to require skill and cooperation to take down. (Trust me, get a group of 100 people to take down 6 things at the same time that are moving, ie body-mounted shield generators on a Kaiju, it will take them a while.) The beautiful thing about this is mechanics can be inserted and trigged by player numbers, and they can be developed, put in, then only used when RNG determines that the "EMP-No-Fly-Zone" Kaiju should come out.
If we're looking for competition, look to charts. Damage done, healing done, people revived, automated turret damage, defense repair values, time spent airborne in combat(silly but whatever), damage taken before death, enemies killed, headshots achieved.
Player versus Player creates a huge wave of variety in of itself as the skill of the players and scenarios they're in can largely effect the outcome. Pure PvE just isn't fun and in today's gaming world doesn't last long. Killing off PvP (among many things) is what killed Firefall.
Not to mention you can have open-world enemies drop different things based on how you killed them.- If you kill an enemy with all torso-shots, they might drop a piece of tech from their head, or if you kill them with headshots alone, a piece of armor from the torso that can be used for research or it could be combined with other tech to modify a crafted item. Skill can definitely be rewarded in the open world, and the Tsi-Hu can and should be able to overcome a wall of gunfire. It just takes some creative design.