As terrain gets more complex, should I focus all my resources on one character?

Focusing your training based on terrain might be efficient, but don't forget to prepare for variables. Flexibility, rather than sticking to one path, is key to victory.

For complex terrain, it's best to see which of running, swimming, climbing, or flying is most overlapped by the terrain, and then choose from your own gadgets that are complex. The same goes for your own stats.

Please try to balance your stats as much as possible. Training bags are random, so it's hard to get exactly what you want, but try to keep them balanced. If there's a specific terrain you're stuck on due to a lack of gadgets, it's okay to focus all your stats on that particular ability. You can decide based on your situation^^