You would probably need to give a little more information for any advice to be of help. Broadly speaking, if you have evidence to raise the fact that the testator did not have testamentary capacity, the burden of proof shifts to the person seeking to rely on the will and they will have to prove on the balance of probabilities that they did have capacity.
As far as undue influence is concerned, this is actually extremely difficult to prove and the burden of proof is on the person alleging undue influence. Normally it is pleaded as lack of knowledge and approval (ie the testator did not know and approve the contents of the will).
As to the chances of successfully challenging a will, that will depend on the evidence available in each individual case, but it can and does happen regularly.