@Capehorn
There are a number of different ways to bring the boss down:
-First method is to wait for his shield to dissappear and try to attack him twice. You have to stay alive at least 13 rounds (1 round = 1 summon) to make that happen. If you don't get greedy and play it as safe as possible, making sure you are always equiped with as many swords as you can, it's doable. Focus on taking out the spawns first to eliminate the possibility of getting cornered, even if that means not damaging him in every round or just damage him once.
-Second method is indeed to try and lure enemies into his shield to disable it and attack him more times. But as you said this is pretty tricky to do (and requires some luck). Supposed you do that in every round you could bring him down in 5 rounds (best case).
-You can ALSO damage him in between rounds and before he starts spawning. This is by far the fastest way, but you need to think a bit out of the box here. I won't get into more detail or spoil the methods yet (yes there are various different ways), just hint that you'll be able to do that safely, once you learn and decipher his moving and attack patterns. Using this you could kill him in 5 rounds or less (at best).
-I should note here that it's (theoretically) possible to even kill him at 2 rounds by combining methods 1, 2 and some of the method 3 variants.
As to what is the "intended" or safest way to do it, well, there isn't any. Each method has it's own risks and rewards. What worked best for me was a combination of the above , depending on the round, move and attack pattern, while always making sure I never took unnecessary risks.
@Malko: ha, ha, well done mate!
Yup, the 2 kills with one sword is a known one
More may even be possible.