You could 5 box an extra Warlock in the slot that the level 13 is on which would put you closer to having an extra summons option when the Warlock hits 20. Once the group is 13 swap the Warlock to the existing level 13. Even if you plan on having a Warlock in your main party, as long as the extra Warlock is on a different slot in your 5box group you could even send your main warlock back to town and summon him back.