I bought a copy of X recently and started resetting for a Timid Xerneas. The method I'm using seems to cut down the time for going through resets considerably compared to other resets I've done, and I haven't seen this posted anywhere. Here's what I've been doing so far:
Synchroniser: Anything with the desired nature
Speed checker: Smeargle 1 point below Xerneas holding an air balloon, and knowing spore/rock polish/recover.
HP checker: Staraptor with final gambit and total HP of 200 (1 less than 30/31 IV Xerneas).
Xerneas is an annoying one at first as there is a cut-scene to watch. You need to run/faint xerneas if you don't get what you're after first time round.
Initial setup:
- Fainted synch in front.
- Smeargle enters first to check speed (see below).
- Staraptor uses final gambit to check HP.
- Unless it's what you're looking for after following the general process below (which it probably isn't), faint/run from xerneas. You'll then get a message saying it wants to help you. At this point, SAVE.
Once you've saved and have the legendary in front of you, the general process is as follows:
IF fairy aura activates before the air balloon message on smeargle, 30/31 IV in speed confirmed.
IF fairy auta activates second, speed on Xerneas is low; RUN, then interact with Xerneas again.
If you confirm speed:
Turn 1: Spore with smeargle.
Turn 2: (Free) switch to staraptor.
Turn 3: Use final gambit. If Xerneas survives, 30/31 HP IV confirmed; if Xerneas faints, SOFT RESET.
Turn 4: Switch to smeargle, try to catch.
Once caught: Watch the annoying cutscene, check stats during Lysandre battle.
Reason this works is that nailing the speed is arguably more difficult than HP. The odds of getting the correct nature AND max IV in speed are less than just getting a max IV in HP. Finding speed out straight away and being able to just run rather than soft-reset saves a lot of time using final gambit, waiting on the HP going down, and then resetting/loading the game if Xerneas faints. As you don't even need to make a move before confirming speed, you're also saving time.
The above should also work for Yveltal.