Complex sentence: During this battle, the tactic of the rolling barrage was employed but despite this, many casualties were suffered in no-man's land during the assault, about 50 percent of the total casualties occurring before the first German trench had been captured.
Split into simpler sentences: During this battle, the tactic of the rolling barrage was employed. However, despite the additional cover, significant casualties were sustained in no-man's land during the assault, with estimates of 50% of casualties occurring there before the first German trench had been captured.