Many of these units can be obtained by clearing in-game stages and are usable in Prime Battle Events. All World Tournament characters are not usable.
These Prime Battle Events also have Legendary Challenge Campaigns and some have Extreme Challenge Campaigns where alot of there Freely obtainable are required to be rainbowed and fully Extreme Z-Awakened. (Check the links for more information about these campaigns.)
(see also: Recruit Cards / Freely Obtainable / Freely Obtainable (WT))