When defending in a PHR remember that no witness evidence is called. As the whole procedure has the aim of placing a financial deposit of up to £500 on the Claimant with a weak case you will often be up against an unrepresented Claimant who has not pushed their case through a legal filter of a Solicitor.
Remember to produce a counter argument for every point raised in the ET1 and know your case inside and out as the unrepresented Claimant is likely to add new evidence at the last minute or refer to something not brought up in the ET1.
Remember lastly your main purpose is to tell the judge why the Claimant has little prospects of success at a full tribunal.
Recently I had to explain why a skiving worker was likely to be held fairly dismissed at a full tribunal after he was found moonlighting when a company employed a PI to follow him. Srangely he was working as clairvoyant and crystal ball reader but hadn't seen it coming!!!