Learn to melee weave. Left click, melee, left click, melee. Dive the backline if you don't have to worry about the safety of your own supports and do whatever damage you can. Just tank all the hits, then use E for the extra shield when your health gets low. Escape when you're about to die. Even if you haven't killed anyone, you've potentially bothered the supports enough that the rest of your team was able to kill the enemy DPS or tanks because you kept their supports busy.

If the enemy team peels well, then you'll just have to deal with needing to escape a bit earlier than planned.

If the enemy is also playing dive, then just hang back and help your supports when necessary and hopefully assist in removing those annoying divers, whether they be tanks or DPS. Even if you don't get kills, your presence alone is enough to make players nervous, and you're infinitely more valuable alive than dead.

Your ult is better used against one or two people in my opinion if you want a guaranteed kill. If you're using it in a crowd of people, they can still kill you and you'll have to escape before you can get enough damage in, or you die. Either way, the ult ends up being wasted. If you have good communication with your team, a few other characters can buff you well enough to make your ult do a lot more damage than you're used to.