i feel much much more comfortable being the defensive / support character (and stock saving cuz my DI *****, true story), so i try to focus on that
you're supposed to be the supporter
shieks only real kill move is that slap, so unless youre edgeguarding/chaseing off the stage
your main goal is to rack up damage and leave it up to your partner to get the kill
presumably being that your partner is fox with those usmash/uair rapesies
or falcon with his knee's/fsmash/dsmash
the only real characters you have to be super aggressive as a partner with is like peach
because peach racks up that damage much quicker with her dsmash and she doesn't have any real kill moves so just slap them homie
source: from my personal experience. I always played passive while luigi played aggressive, but when i teamed with jtb/brian i played the aggressor. also really depends on the matchup/chemestry boss plays much more aggressive than jtb so i played passively with him.
i always found myself placing quiet well in teams regardless of my teamate but you'll probably ignore my opinion unless i tell it to you in person
also: very important to read tech rolls.
as far as teamates go, try to choose a fast faller. From my experience and observation the best teams are floaties with fast fallers. Floaties tend to rack up damager quicker and live longer so they are ideal for the passive/stock preserving roll while fast fallers tend to have da broken smash moves to get the kills so as long as you hold stocks/rack up damage they can take the risks necessary to kill the other teams stocks.