Complex sentence: The British did not have time for a lengthy formal siege and so Macdonald proposed that feints would draw Tibetan soldiers away from the walls over several days before an artillery bombardment with mountain guns would create a breach, which would be stormed immediately by his main force.
Split into simpler sentences: The British did not have time for a lengthy formal siege, so Macdonald proposed feints which would draw Tibetan soldiers away from the walls over several days. An artillery bombardment with mountain guns would then create a breach, which would be stormed immediately by his main force.