Effectiveness is, in simple terms, how well something works.
For example, in legal, when you compare international law and domestic law, you could talk about how quite often, domestic law can be considered quite effective as opposed to international law, primarily due to its enforceability and the binding nature of statute law (something alone those lines).
As your question is asking you to "assess" how defences can achieve justice, just delve into different types of defences - you have your complete defences [as there are so many split them into paragraphs - e.g. PARAGRAPH ONE: duress & necessity & PARAGRAPH TWO: insanity & self-defence etc...] as well as partial defences (i.e. diminished responsibility + provocation].
When you talk about achieving justice, think about it from different views (i.e. not only justice for the victims but perhaps also the wider community - maybe even the suspect themselves (i.e. are their rights being upheld?)
In relation to how you would assess effectiveness - you could just focus on cases & legislation - if they are achieving justice, state that they do indeed increase effectiveness of the criminal trial process and if not, say the opposite.
I hope that helps