If we want the revered guardian to be able to survive and win the challenge there are a few things we need to look at. These are: Weapon skill, Strength, Toughness, Wounds, Initiative, Number of Attacks, and Saves.
He already has a decent weapon skill of 4, but with only three attacks it is not very likely that he will be able to hit enough times to actually kill a combat hero/lord. Giving him the sword of striking, +1 to hit will give more hits to convert to wounds. The Shrieking blade, could reduce his opponents WS to 1 IF they fail a Ld test (not that likely with characters) with either sword he will not have the bonus to strength from the halberd but will have an increased AS.
With a strength of 4(5) he will not be able to accurately determine wounds as the toughness of combat characters varies, UNLESS we can increase his strength to a point where he will wound most things rather easily. Things that are less than 25 points that will increase his strength include: Sword of Might (lets us increase out AS by one as we can now use our shield in CC, however with a halberd we already have S5.) And the only other item available would be the potion of strength, with this item we can get S8 for one turn, that is enough to wound on 2's versus everything.
Note that very few characters have a toughness greater than 5 so we can assume that wounding on 4's will occur most of the time. if you frequently come across those characters the relic sword might be an option, always wounding on a 5+ (but even then I wouldn't recommend it as with a halberd you are wounding on a 5+ anyways, the only advantage is the +1 to AS)
A toughness of four is not bad, BUT we see MANY characters with a higher strength than four so our Revered Guardian will very often be wounded on a 3+ or worse on a 2+, with a single wound this means that we need to make this save, which is at least -2 to armour. Saving on 6's is no way to guarantee the survival of the champ. If we can increase the toughness we have less chance of having our champ wounded so we can give him the potion of toughness, +3 toughness means that he will be wounded with much less frequency.. I don't know of any other item to increase toughness so I will leave it at that.
I am not familiar with any way to give the champ an extra wound so this point is pretty much moot.
With initiative Saurus he will be hitting almost every character last, unless they have a great weapon (and not High Elf) I know of no hero, let alone lord, type characters who would be hitting last. We can augment this by giving our champ either the sword of the hornet or the sword of swift slaying. Both give him the always strike first rule which is useful against goblin big bosses and maybe skaven cheiftans, as you can actually hit enough to cause enough wounds to kill them (remember you are now at S4 with a 3+ AS). The benefits of this would be that you can at least cause a wounds on some characters (striking simultaneously against High Elves is always nice) and has great synergy with the lore of life (do a wound and die, res and repeat). The gold sword gives an initiative of ten which means only characters with ASF will hit first (they will never get re rolls though), this also reduces S to 4 but increases AS to 3+. The last item to increase initiative is the potion of speed, at +3 to I he will be striking in respective order, but is one use only and no sure way of guaranteeing striking first (it is pretty cheap though).
With more attacks there is an increased chance to hit, wound, and kill. Several things can increase numbers of attacks, the Sword of battle gives an extra attack (Increases AS as well, but lowers S). There is also the potion of Foolhardiness which gives you the Devastating Charge and immune to psych for a single turn, if you think you will be getting the charge than you can consider this (not the best option I must say). Any item that gives frenzy is also worth considering, such as the Berserker sword (S4 3+AS) or the carnosaur pendant (although he must have caused a wound first)
With three types of saves there are many possiblities. We could give him the Dragon Helm for a 3+ save AND a 2+ Ward against flaming. Or the gamblers armour for a 3+ save and 6+ ward all the time. Charmed shield gives a 2+ ward against the first wound suffered. The Dawnstone allows for re rolls to AS, opal Amulet gives a 4+ ward against the first wound suffered. Talisman of protection instils a 6+ ward. Seed of Rebirth gives a 6+ Regen. Luckstone allows ONE AS re roll, but is very cheap. If you expect flyers then it might be worth considering the Pidgeon Plucker Pendant, for a 5+ ward against flyers. The Other Tricksters Shard is useful if you expect your champ to survive the initial onslaught and get some attacks in, but only if the opponent has a ward save.
And that covers it for magic Items. So what to do? Consider how you can get some synergy in, I have already mentioned the ASF swords and Regrowth, but there are many more. Such as Speed of Light, for I10 and WS10, and anything that increases S or number of attacks. Also Birona's Timewarp and potion of Foolhardiness, with M8 +2d6 you have a very large charge range, a champ that strikes first AND has 5! attacks (on the charge).
So, long to short, the best way to get that underdog victory is through the synergy of magic and the item selection.
Let's now start listing a bunch of combinations, and if anyone wants to crunch the numbers VS as Standard a combat hero/lord as possible that would be great. (sorry for the long post)