I was unimpressed by the performance of Dr Strange in Avengers - Infinity War. He seems to have underused his power as noted in other questions (and responses pointing out why he did not use them).
Having seen Loki stuck between two portals for 30 minutes in Thor Ragnarok I think that this could be yet another way for Dr Strange to get rid of Thanos: opening a portal and letting him travel to the other side without opening that other side.
I was wondering, though, whether spells stop when the caster dies? (this solution would buy at least some time for the Avengers / Guardians to work out a better plan).