You will kill the whole party
Pathfinder 2e does not balance encounters the same way as Pathfinder 1e, or D&D 5e, or any other RPG I'm aware of (D&D 4 might be close, but I haven't played it myself). A single creature 4 levels higher will be a very difficult fight for a party of 4. Any single enemy 10 levels higher will be essentially impossible to fight.
The guidance on multi-leveled parties is:
If you choose not to keep the whole group at the same character level, you'll need to select a party level to determine your XP budget for encounters. Choose the level you think best represents the party's ability as a whole. Use the highest level if only one or two characters are behind, or an average if everyone is at a different level. If only one character is two or more levels ahead, use a party level suitable for the lower-level characters, and adjust the encounters as if there were one additional PC for every 2 levels the higher-level character has beyond the rest of the party.
Their recommendation is to make the combat level 10 (or higher for increased difficulty), but for 9-11 (4 level 10, +5 for the level 20, +2 for the level 15) characters. It does include that you can use the 'average party level', but I believe that is written with the assumption that the party isn't split between such a large gap and the 'single higher character' model works better, even with 2 high level characters (especially since they are Not the Heroes).
I'm not sure how you calculated that a level 25 enemy is a severe encounter. Even with your suggestion of using the average level of 13, a level 17 enemy would be an Extreme Solo Boss fight. However, the level 20 character could handily solo such a 'Low or Moderate threat lackey'. Most Adventure Paths so far end with an enemy around level 22-23 with a level 20 supporting character for a group of level 20 PC. A single spell or ability from a level 25 creature (say, horrid wilting/desiccate at 10th level) could annihilate some or all of the level 10 characters. With Treerazer's DC of 49, any low level character that didn't roll a natural 20 would take effectively 24d10 damage. Meanwhile, the party will have nothing meaningful to do except provide bonuses to the level 20 character in the hopes they will get lucky.
Consider, instead, an old trick of adding NPC's to parties - the cinematic clash. The level 20 (and possibly level 15) NPC go to fight the BBEG somewhere visible but not close enough for the party to be affected incidentally. However, said character(s) will 'surely be overpowered' if the BBEG general, a boss in their own right, isn't held back/defeated by the party. Create an interesting boss fight for the party, and provide a bigger threat for the higher level characters to be busy with. It could even be that the party is fighting the BBEG, but they've summoned something beyond their control that only the NPC's are a match for ('Summon the Kraken!'). Make sure you're clear that the NPC's are going to handle the over-deadly encounter so that the party is free to disrupt the BBEG/summoning ritual.
