{Restoration Angel}
{Silverblade Paladin}
I'm at 3 life. Opponent at 7.
Paladin has 2x {Ethereal Armor} and {Rancor} attached to him.
1.) I enter combat.
2.) I declare Paladin as attacker.
3.) Opponent declares a spirit token to chump block.
4.) In response, I cast Resto Angel(not flickering anything), and choose to soulbond it to the Paladin, giving him double strike, and thanks to Rancor, winning.
My opponent claims I can't soulbond because the paladin is already attacking. I say that since we didn't enter the first stroke phase of combat yet, that this works. Rulings?
You're fine, since the combat damage step hasn't begun yet. Here's your ruling:
510.5. If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
RESOLVED
Also, players receive priority after attackers are declared (rule 508.3), and before combat damage. At this point you may cast spells and activate abilities.