Well, it probably depends heavily on said horde's composition - an IG horde plays very differently to an Ork or 'nid horde.
With IG, they're likely to stay still as much as possible and simply shoot you from afar. And, with heavy weapons, orders and many surplus bodies, they may well win a shooting war.
On the other hand, Orks and nids will basically have to come towards you and, whilst their firepower is far from useless against AV10, combat is their main goal.
For all 3 - and IG in particular - monoscythe missiles are your friends. Grotesques are good against IG and might work against orks and nids too. However, I'd say be wary of Ard boys and/or Pain Boys - those 4+ saves and FNP make them a lot more durable against grotesques than you might think (I've learned this the hard way). But, if they don't have those... have fun.
Also, be wary of poison on nids - especially since they'll be striking at the same time (possibly faster) as your Grots.
For IG, I'd say taking out their commanders are the key (especially since they tend to have far fewer surplus bodies). Removing their ability to ignore cover or reroll failed armour penetration rolls really hurts them. They may also dislike DA - since their basic weapons can't touch that T7.
With 'nids, if they don't have adrenal glands, then they can't touch your AV10 vehicles - so just stay inside and shoot them to death (though I'd advise first shooting at anything that can
touch your vehicles).
Anyway, in all 3 cases, I think the best route tactically is to use our mobility as much as possible. Against Ork and nid hords, clustering up will cause them a lot of trouble as all those infantry will be tripping over themselves trying to reach you (and, in all probability, creating even better targets for monoscythe-death).
Similarly, with IG, try to position yourself so that their heavy weapons can't see you (but preferably so that you can still pick off some infantry). If they're forced to move and snapshot, it should utterly cripple his firepower.