Cards that instead of dealing damage, will remove life points. So for the Rescue mission #4 you don't necessarily need to use Rescue. You can use Beserk cards, or just lose 35 times with C Wing. As long as the card is -opp life.
Thanks @0 spring and if anyone looks at this post even though it is answered, is there a way to make a good amount of clintz?
Yep, play a lot. Mainly DT and Survivor mode.
If a mission dosn't state a clan then you can use any cards so "- 20 opp life" can use any cards. "-20 opp life with Berzerk" needs Berzerk cards.
p.s This will help with clintz: http://www.urban-rivals.com/en/community/forum/?mode=viewsubject&forum_page=0&id_subject=2108439&subject_page=0#answers