Kinda been covered already, but basically, Metro is a Legendary hybrid. Magna is a Unique superhybrid. Magna should be stronger than Metro, but the reality is that it isn’t actually that much better. Metro does less damage, sure, but it’s also considerably tankier–not only does it have more health, it also has better Rend resist, which is pretty pertinent given the abundance of DC. Magna’s biggest problem in arena is Swap-Ins. Metro has far less issue with that. Also, once you account for the discrepancy in Health, Metro & Magna’s stats when boosted are pretty comparable, meaning you’re basically looking at the difference in a better Crit & T3 but less Rend resist for Magna… but considering how these creatures are typically built, you’re not likely to get a T3 unless you’re revenging, and if you’re revenging, there’s a decent chance that Metro can just Strike on T1 and preserve a good T3.
Metro is just too close to Magna for what it is. It needs more-clearly defined weaknesses, as it claps both Fierce & Resilient and doesn’t do too shabby even against its own counters (Cunning), especially in boosted where it can often put a lot of Cunnings in swap-in range with a single hit. One of the problems that people run into in arena is that it’s very difficult to beat Metro even with a counter unless you’ve invested a lot of levels & boosts into the counter, and unfortunately, the abundance of Resilients make investing in most Cunnings that hard not very worthwhile.
All that said, I don’t think Metro needs a big nerf. I would simply remove its Distraction resistance–neither component has it, and it would let Cunnings walk away with a bit more health against a Metro. Metro would still be very strong, but it would simply be more practical to run Cunning creatures to deal with it.