Best stand-off weapon for...

[Wdll]

I don't remember the last time I had to fight against digged in entrechments enemy infantry, but in general against infantry I have found the absolutely best weapon to be CM artillery. It obliterates them.
You can always use artillery (any) to suppress, smoke infront of the trenches, bring in infantry together with some that can see through the smoke and leave them near the enemy, then with the TI IFV/APCs/Tanks, suppress suppress suppress and move in with your infantry. It will not be as casualties free as some other approaches, but it will get the job done much sooner.
If the enemy does not have scary anti tank units (of the infantry type), then (though not sure about USA), if you have light tanks which are cheap, just send them outside the range of the enemy inf RPGs and let them feast.

If the enemy air defence is not that dangerous, bring 1-2 Longbows and have them to scout, they work wonderfully for that, just make sure you don't let them go first and have any nasty surprises from hidden enemy inf.

[Lt. Ketch]

I like using 40mm AGLs. They're small and can suppress rather well. The ones on hummers can also be used to get your small squads places or to pickup suppressed units. Use direct fire for suppression. Their HEAT rounds can also take out some APCs. I had one kill three BTR-80s in one turn