At Soul Level 120 your quickest route to Sunbro-ery is probably going to be helping people in NG+ out with
and the
. Of the two, the former is probably going to be the easier. Your Soul Level puts you out of range for most people who haven't reached NG+, and most people only summon help for the hard bosses after their first playthrough. You might be able to get some NG+
action, but you'll be waiting longer since it's an optional boss. In NG+ it's an HP sponge, but otherwise not too difficult if you know the pattern. The only other highly active co-op area at that Soul Level in my experience is
, but some of the people summoning you there will want your help for the level but not the boss, and will be sending you home when you reach the fog door. Probably not your best choice if you're trying to rack up Sunbro wins.
Invasions on the way to the boss are extremely common. It's usually in your best interest when co-oping to pack a set of PvP gear and be practiced in swapping to it quickly, but if you're not planning on doing much co-op and haven't practiced in PvP you'll actually probably fare better using what you're used to. Just remember that virtually all invaders anymore are looking for a backstab, so don't give it to them. Slam your back against a wall with your shield up if need be; if they get impatient before you do you're at an advantage. And remember that if someone invades before the host summons you there will be no warning that there's an invader on the loose when you enter the game. I just assume every time I get summoned that there's an invader lurking somewhere and constantly scan for red and blue glowing figures.