At this time as you can see Leo is also present inside of the portal and he already killed all those Monsters that were close to that portal gate. Well, because this was just and Early epic grade portals naturally it was very easy for him to kill those Monsters.
It would be better if the number of those Monsters wasn't that much. Naturally because of the number of those Monsters it takes him that much time to kill them. But even with that he already cleared one portal, and he already came inside of the second portal. Fortunately that portal was just about to get completed, so still no monster came out from that and before they could come out he already went inside of the portal.