I'm fishing an event as a CoAngler. I have all my combos set to go and want to throw a casting reel in my duffle bag in case one of mine goes down and I need a backup. What kind of line should I put on my backup reel so that it would be ready to go in any situation?

I'd use 30 lb braid. 

You'll be good for every presentation except punching. 

If you brought a designated punch outfit with you and that one goes down, just take 10 min and transfer it over to the spare. 

 

 

 

 

Mike

braid and your choice of leader would probably be the most versatile.

 

that being said, id throw some 12lb yo zuri hybrid on it but im prob in the minority.

30-50lb braid is my choice. With a leader. That way you have all your presentations covered. If not braid them 12-15 yo-zuri or p-line cxx

50lb braid

Or 15 lb big game

When I was co-angling, I took 2 spare bait casters.  One filled with line appropriate for reaction bait presentations and the other for "feel" bait presentations.  Much more often than not, it was the crank/spinner bait/square bill reel that I would back lash.

3 minute rule, i.e. if I couldn't get the tangle out in 3 minutes or so, cut the line, store the tangled reel and bust out the alternate.

Don't forget spare spools for whatever spinning gear you bring also.