Surprisingly, I don't think Liz would actually be unstoppable on Earth, just incredibly dangerous. Amber is a whole other story, and would make a good power match-up absolutely necessary to even stand a chance.
Liz appears to be a glass cannon. She doesn't seem particularly durable, hasn't really displayed any superior physical strength, and notably can't regenerate on her own. However, she can instantly drag someone out of the fight and set up Amber to go to town without having to generate her own water for usage. It seems utterly broken, like a more balanced version of Nico's powers. But her teleportation does actually does have a decent handicap to it. She has to get up close and personal to use it, which is risky when you can't heal on your own, and it actually has travel time over long distances if she or a passenger is the thing being teleported. Her tactics in the Monster World were high risk and high reward thanks to the unique and deadly water there, but on earth that trick is very much punishable if you know it's coming.
Amber would actually be without a doubt the most dangerous twin on earth. Her ability to manipulate and generate water makes fighting her a near guaranteed death sentence if she traps you in a sphere of water, her ability to absorb MP makes her borderline inexhaustible, and to top it all off she has MC-level regeneration. I'd go as far as to say it's actually better than the MC's was at that Level, because headshots don't seem to affect her ability to fight at all. She's only barely saved from being a complete story breaker by having a slow absorption rate for MP. Due to it she can't actually stop energy-based attacks, and using her water form drains her MP faster than she can replenish it. If a superhuman isn't at least Level 2 and have some good powers based on some type of supernatural or natural type of energy, they've already lost.
