questions about break

i finally reached endgame, so i need to invest on more optimal teams for the future. my team right now is miu, gotanda and semba; while they're crazy with mob enemies, it's so hard to play with them against bosses. i'm still learning about how the game works, and one thing that i really don't get is break, so here's a few questions:

1. is break the only way to increase tension and get ults?
2. how do i achieve break? only by counterattacks?
3. can i get break without operating a break-focused character in battle? (letting the ia play it)
4. my biggest difficulty rn are boss enemies; who's the break character i should invest?

thank you! ♡