If you play one-on-one, then you do not have to worry and don't make a call with air or play a coin-flip. Just wait for your hands to be above average and see how your opponent reacts to limping and raising. If he constantly goes all-in on your limp, then you need to limp only with strong hands. If he sometimes goes all-in on your raise or folds (for example 50% 3bet push, which is considered the optimal game for 20bb), then raise with air and with good hands that can a call to all-in. These are all my recommendations for playing in the HU SNG, I do not play other type of tournaments.