How to change what version of ACIV the companion app syncs with

The agile guards use throwing knives as weapon, of which you can only carry 1 at a time. If you drop their weapon to the ground, it's most likely you already carry a throwing knife. If that's the case, just kill the guard with the knife you already have.

The best way to finish the challenge is by doing the following:

• Equip your fists (unarmed) and the throwing knife (it doesn't matter if you've got one or not)
• Initiate combat with an agile guard (they are the ones that don't wear hats)
• Wait for them to attack, then disarm them by pressing counter followed by break defense.
• Press shoot to throw your knife, killing the guard in the process.
• Rinse and repeat.

I've never had much problem. I have various methods of climbing ships.

Usually if I have to capture a flag my tactics are after boarding starts

1. Use the swivel gun to do what damage I can.
2. Use the quick elevator thing on my own ship to get up to the platform.
3. Shoot enemy soldiers on the central platform (or others).
4. Swing across to their boat on a rope and assassinate people and then fight generally til I am at or close to the soldier death limit (if one or two short I hope that my crew will finish the job).
5. Use the quick elevator on their ship to start my ascent. There is of course no soldiers up there any more.

This has never really failed me. I do occasionally skip some steps but the above is, in my view, pretty efficient and pretty foolproof.