How to claim meal and entertainment expenses when it falls under the 50% rule?

That depends on who is asking. Is this personal, or corporate? (could also be partnership, trust/estate)

Is this a personal tax query? If so, you can record the full amount as meals and the HST as HST ITC, then at year end make an adjustment to record half of the amount as meals expense on the T2125, and remove half of the ITC claims for meals that are at 50% (some meals might be at 100% because you've been reimbursed for example).

There is a special dispensation to allow you to make this adjustment once a year, it's found in the GST General Guide. 

The half that isn't claimable, and the half of the ITC gets recorded to the proprietor's draws account at year end so you can reconcile your income statement against your T2125.

If this is a corporate tax query, you would also record the full amount as meals expense, and the same with the HST, adjust at year end. When you adjust at year end, reduce the ITC claim and record that 50% amount as non deductible expense. When you record your income on the top of the T2S1, you include 100% of the meals and the 50% of the HST and add back 50% of meals and 100% of the 50% of the GST/HST on the form to increase your income for tax purposes, but you don't do anything with it for accounting purposes. No need to charge the shareholder loan account for that amount. 

The T2S1 has many lines on it for many adjustments from income per the P&L to income for tax purposes. It's very different from the T2125 for personal purposes which requires you to complete each line, and requires 100% of meals to be recorded and then reduced by 50% to arrive at taxable income.

Hope that helps. Take a look at the tax forms, just google T2125 and T2S1 to see the required reporting. And yes, you do report each line for tax on the T2, on the S125 using GIFI codes.