Merlin is the heavy hitter of a Grail Ice Suite. It is expensive to rez at 6 and because many players face-check using a sentry breaker they may not be prepared to get damaged by a code gate.
Merlin can support or be supported by Lancelot for program desctruction and/or Galahad for End the run. The best support for Merlin is another Merlin, or even better, two Merlins, you might even be able to get a flatline. The most likely way you can get another Merlin in your hand is to use this in The Foundry: Refining the Process decks. Another way is to tutor for it with cards such as The Future is Now, Project Atlas or Levy University. Now levy will give the game away, but it does force them to get all three breakers (or a good AI breaker) out before running unrezzed Ice again (alternatively they could just run with 4/6 cards in hand).
Another less likely possiblity is Excalibur which may or may not be relevant depending on which server you have located your Ice on. Often central servers get run 1st in multi-run turns, either to deactivate Jinteki: Replicating Perfection, build up Datasucker tokens, during a multiple-run Medium dig or sometimes during a Account Siphon/Vamp and then run unrezzed Ice play. Defending a scoring server with Ash 2X3ZB9CY, Caprice Nisei, Red Herrings, a Nisei MK II agenda token or even Marcus Batty becomes much more effective when it is their owly run that turn.
Grail Ice will take up a big chunk of influence in whatever deck it's played in. Decks that synergise such as The Foundry: Refining the Process or possibly NEXT Design: Guarding the Net can support grail Ice and The Foundry in particular is very synergistic since it lets you pull additional Grail Ice to your hand.
In general grail Ice is not particularly good because of the following drawbacks:
Moderately high rez-to-strength ratio
Influence hungry neutral cards (except for Excalibur)
Require additonal Ice to sit in the hand until the runner runs on a server containing rezable Grail Ice. The additional cards clog up the hand and compete for slots with Agendas, Operations and even installables that are not quite ready to hit the table.
The Corp has limited deckslots for Ice and usually needs to defend several servers including Centrals, the scoring server and possibly Asset economy, Ice that stays in the hand may become a temporary or permanent 'dead draw', especially if the runner is not running the relevant servers.
Ultimately I do not recommend using Grail Ice except for #Excalibur, which is a superb piece of Ice for many types of decks. I tried using it and it's simply too easy to break and not much of a threat once surprise is lost. Since there is no point 'splashing' Grail you will need to commit half your hand and half your influence to it - yes it can be fun to play with, but not in any deck you want to regularly win games with.
TL;DR to double the number of subroutines. Since they are both 'when' constant abilties you should be able to trigger them in the order of your choice.
Marcus Batty to try and trigger one of the gained subroutines.
Mumbad City Grid to keep adding additional subroutines.
Snare!, Shock!, Shi.Kyū or even Fetal AI in your hand to finish the job. House of Knives, Hokusai Grid or a next-turn Neural EMP/Ronin/Philotic Entanglement score can be the finishing move.