Yeah, most freezers, bombs and healers are either one use (gone forever after used) or once per battle (gone but will be back your next battle). So with those you have to predict when they would be the most useful and hope that you're right. Knowing what your opponent can use, using TDN's 1-player profiles page, which can be found here, https://thedailyneopets.com/battledome/secret-challengers under the Active Challengers heading. Can help you decide what defensive items you may want to use, depending on what weapons they can use.
In the case of Holiday Memories. You'll get the 20% heal not matter what, but whether or not you block any damage when you use it, depends on what your opponent uses and if they have those icons that can be blocked. So yeah, it's basically luck and insight.