This was rare (Savannah Lions), then it was uncommon (Elite Vanguard). It's never been common before. It's probably not broken at common, but it will definitely impact limited.

Yeah it may be too strong at common, maybe a drawback like CARDNAME can't block? or attack each turn if able?

Heh, and now it gets a Goblin Cadets-style drawback that makes it a heck of a lot worse. But then again, even Goblin Cadets saw some play in hyper-aggressive decks; this is probably still playable in certain of the most aggressive limited and constructed decks. Which is a neat place to be: good job.

I tried many variations to keep this a 2/1 for common, this one worked the best. Glad you like it.

I don't like its text. You have to give a creature an ability and creature a new combat step just to get this killed. "Whenever this becomes blocked, sacrifice/destroy it." would be much better.

This is the way it is because, whilst it is a strong drawback, it can be played around with pumping cards/abilities. Your suggestion would just kill it outright which would make it worse.

Also, it does nothing to the combat step as the blocking creature gains the First Strike upon being assigned which I feel is quite straightforward.

I think SU was pointing out that if no other creatures had first strike, then the first strike combat damage step wouldn't have happened. But I don't think that's enough to worry about in my opinion. Compare with recent creatures like Witherscale Wurm.

Witherscale Wurm is rare. :P

Okay, I just meant to say that giving first strike is a bit of an awkward hoop in case you don't have a pump effect. Or I might just be thinking about nonexperienced players too much.

In either case, I think the standard would be "Whenever Sandrunner Lion becomes blocked by a creature, that creature gains first strike until end of turn." Longer, but a lot of things in Magic give you the ability to respond no matter how irrelevant (see the Wurm), so this shouldn't be an exception.