Ketan -
Although you should focus on the conclusion to weaken or strengthen, you need to take different approach for each question type.
Weaken - You can weaken in possibly two ways -
1. Provide alternate explanation - Usually, when you see argument about comparision (e.g. an artifact was found in Eurassia but the stone was native to persia....), single premise based,predictive, etc, you need to find an alternate explanation. The purpose of this choice is to simply discredit the conclusion by showing that there is another interpretation.
2. Provide additional evidence - Usually, when you see casual argument, look for additional evidence. The additional evidence simply disorient the conclusion. For example, If the conclusion is that the bank failure caused the interest rate to go down, you need to find another evidence such as "the unstable stock market forced investor to withdraw money from bank leading to bank failure and lowering interet rate".
Strengthen - Strengthen is a different ball game. Although you need to focus on the conclusion, first state the assumption. Also, once you pick the answer, connect the conclusion and the answer choice using BECAUSE to make sure that the answer does strengthen the conclusion. There are other tips to ensure the validaty of the strengthening choice.
